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A problem unstuck? Evaluating the effectiveness of sticker prompts for encouraging household food waste recycling behaviour.
Linzi Shearer a b *, Birgitta Gatersleben c, Stephen Morse a, Matthew Smyth b, Sally Hunt b
a Centre for Environmental Strategy, University of Surrey, Guildford, Surrey, GU2 7XH, United Kingdomb Waste Development, Surrey County Council, Kingston-Upon-Thames, KT1 2DW, United Kingdomc Department of Psychology, University of Surrey, Guildford, Surrey, ,GU2 7XH, United Kingdom
Abstract
This Randomised Control Trial (RCT) investigated the effectiveness of using stickers as a visual prompt
to encourage the separate collection of household food waste for recycling in two local authorities in
South East England. During a baseline period of up to 15 weeks, separately collected food waste was
weighed (in tonnes) and averaged across households in both treatment (N = 33,716 households
within 29 defined areas) and control groups (N = 30,568 households within 26 areas). A sticker
prompt was then affixed to the lids of refuse bins in the treatment group area only. Weights for both
groups were subsequently measured across a 16-week experimental period. Results showed that, in
the control group, there was no change in the average weight of food waste captured for recycling
between the baseline and experimental period. However, there was a significant increase (20.74%) in
the treatment group, and this change in behaviour persisted in the longer term. Sticker prompts
therefore appear to have a significant and sustained impact on food waste recycling rates, while being
simple, practically feasible and inexpensive (£0.35 per household) for local authorities to implement
at scale.
Key words:
Visual prompts, household food waste, recycling behaviour, nudge, behaviour change.
* Corresponding author
Email addresses: [email protected], [email protected], [email protected],
[email protected], [email protected]
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1. Introduction
Changing patterns of human production and consumption in industrialised nations have resulted in
increased levels of household food waste (Parfitt et al., 2010). In the United Kingdom (UK),
households are responsible for generating around half of the 15 million tonnes of food and drink
waste that is produced each year (House of Commons, 2015; WRAP, 2011). The EU Landfill Directive
(1999/31/EC) specifies that Member States must reduce levels of biodegradable waste sent to landfill
to 35% of 1995 levels by 2020, but does not prescribe the treatment options, collection systems, or
other policies that should be introduced to meet these targets (Defra, 2011).
In the UK, local authorities are responsible for the collection and disposal of biodegradable waste,
which has traditionally been sent for disposal to landfill sites (House of Commons, 2015). Methane, a
greenhouse gas far more potent than carbon dioxide (CO2), is released when biodegradable waste
(which includes household food waste) decomposes anaerobically at landfill sites (Graham-Rowe et
al., 2015). The UK Waste and Resources Action Programme (WRAP) (WRAP, 2011) estimate the
annual environmental impact of manufacturing, distributing, storing, using and disposing of edible
food and drink in the UK to be around 17 million tonnes of CO2 equivalent.
If collected separately from residual waste (refuse), food waste can be used as a feedstock for
anaerobic digestion (AD), a ‘recycling’ process that produces methane-rich ‘biogas’ that can be used
to generate renewable energy, and ‘digestate’ which can be used to produce agricultural bio-fertiliser
(Zhang et al., 2007) and has a lower disposal cost (approximately 50%) than landfill (Nomura et al.,
2011). Households are issued with a small food waste bin (known as a ‘caddy’) for use inside the
house and a larger caddy that is stored outside. The purpose of introducing the service is to
encourage households to separate their food waste from their refuse and store it in their indoor
caddy before transferring it to the outdoor caddy in advance of their weekly collection day. By
diverting food waste in this way, local authorities can increase their overall recycling rate, while saving
money and improving their environmental performance.
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When the first local authorities in the UK introduced separate food waste collection services in 2006,
just 1% of household food waste was being collected separately (Defra, 2015). By 2013/14, more than
half of the local authorities responsible for waste collection in England had introduced some form of
food waste collection scheme (House of Commons, 2015). As a result, the total amount of separately
collected food waste in England increased from 118,000 to 290,000 tonnes between 2010 and 2014
(Waste Data Flow, 2016). Despite this dramatic improvement, almost a third of the refuse waste
stream in England is still composed of food waste (WRAP, 2011). This suggests that many households
are still not participating in the scheme, or those that are taking part are not using their caddies as
effectively as they potentially could be. Since the successful management of any household recycling
scheme is dependent on the effective participation of a sufficient number of households, local
authorities must introduce policy interventions designed to encourage public participation (Karim
Ghani et al., 2013).
To encourage food waste separation behaviour, local authorities have a number of policy
interventions at their disposal (Dahlén & Lagerkvist, 2010; Steg & Vlek, 2009). The decision about
which policy interventions should be introduced is not a simple one for decision-makers as they must
be clearly effective, in the sense of producing changes in behaviour that are sustained in the longer
term (Steg & Vlek, 2009). An intervention must also be practically feasible to introduce and there
must be a sufficient means of accurately monitoring the impact of the intervention to understand
whether it was successful or not (Steg & Vlek, 2009). They must also be cost efficient to implement as
local authorities in England are currently under increasing pressure to deliver ‘more with less’,
following a 40% reduction in funding from central government (LGA, 2014). Hence there is an obvious
attraction for the use of relatively simple and cost-effective approaches.
An emerging body of literature has advocated the use of ‘nudge interventions’ as alternatives to more
traditional forms of behavioural intervention (Dolan et al., 2010; Thaler & Sunstein, 2008). Nudge
approaches have been hailed to be a potentially powerful, low-cost set of tools for policy makers who
are faced with addressing the challenge of weighing environmental sustainability against fiscal
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responsibility when making decisions, particularly during times of austerity (Dolan et al., 2010; John,
2013; Kallbekken & Sælen, 2013).
Nudge is a valuable theoretical framework that summarises ideas drawn from the field of behavioural
economics, which itself draws insights from the field of psychology (Kirakozian, 2016). Proponents
argue that traditional policy tools have ignored the fact that much human behaviour is automatic in
nature (Michie, 2015), recognising that behaviours most often occur as a consequence of both
automatic and reflective processes running in parallel. Nudge interventions, therefore, mainly target
the automatic system and seek to change the ‘choice architecture’ of individuals to encourage
changes to attitudes and behaviour (Sunstein, 2015). The approach assumes that people will rely on
past ways of thinking and acting unless they are encouraged to act or think differently. The options for
changing behaviour centre on providing reminders and cues that both recognise where the individual
currently is while also placing them in a choice environment.
Good designers of nudge policy interventions can steer individuals down new decision pathways
without them necessarily noticing that it is happening. Behavioural change is achieved by altering how
individuals view the attractiveness of an alternative course of action by improving the messages they
receive or the opportunities they have. While no “precise, operational definition of nudging”
(Martaeu et al., 2011: 263) currently exists, a taxonomy of interventions published in a recent House
of Lords report (House of Lords, 2015) described nudges as being any one of the following: changes to
the physical environment; information provision; changes to the default policy and the use of social
norms and salience.
One form of nudge intervention, ‘visual prompts’ has had a wide applicability within a variety of
behavioural fields. Visual prompts are a form of informational intervention designed to stimulate
action or serve as a reminder to engage in a behaviour that might otherwise be forgotten (Chui,
2015). Visual prompts usually take the form of posters, signs, stickers or flyers (Bartram, 2009), and
display factual or persuasive information, or provide cues to aid behavioural decision-making (Geller,
1982; Sussman & Gifford, 2012). Their intensity can vary from simple notices that raise awareness or
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provide procedural information, to more comprehensive statements that provide context and
rationale (Tucker, 2001).
Several studies have demonstrated the effectiveness of visual prompts as a means for encouraging
transportation (e.g., Cope et al., 1991; Huybers et al., 2004) and health behaviours (e.g., Amass et al.,
1993; Andersen et al., 2012). They have also been used to encourage certain pro-environmental
behaviours, for example: litter reduction in public places (Baltes & Hayward, 1976; Geller et al., 1976);
increasing workplace recycling (Austin et al., 1993); and reducing household energy consumption
(Sussman & Gifford, 2012; Winett, 1978).
The effects of prompts on general household recycling behaviour specifically have also been widely
documented, but results are not consistent. Some research has shown that single prompts in isolation
can be an effective way of increasing recycling (e.g., Arbuthnot et al., 1976; Burn, 1991; Jacobs &
Bailey, 1982; Oskamp, 1995; Spaccarelli et al., 1990), while other studies suggest that prompts are
less effective than other types of intervention (Goldenhar & Connell, 1992; Schultz, 1999; Werner et
al., 1998; Witmer & Geller, 1976). A recent meta-analysis found that prompts were one of the most
effective intervention types for encouraging pro-environmental behaviour (Osbaldiston & Schott,
2011). However, as 78% of the studies included in the analysis tested interventions in combination, it
was not possible for the authors to make definitive conclusions about which interventions were most
effective in isolation. It is therefore possible that prompts are effective but only when delivered in
combination with other types of intervention.
Other research has explored the elements of design that can improve the effectiveness of visual
prompts. Several authors have indicated that ensuring the final product is noticeable, simple and clear
is important (Sussman et al., 2012). Adding pictures to written information may also improve
effectiveness (Roberts et al., 2009), provided the images used are congruent with the text (Jae et al.,
2008) and they do not ‘cloud’ the message (van Meurs & Aristoff, 2009). Some studies have shown
that certain attempts to persuade using visual prompts can cause individuals to protest and engage in
undesirable behaviours (Sussman & Gifford, 2012). This phenomenon, which threatens the perceived
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freedom of individuals, is known as psychological reactance (Brehm, 1966; Dillard & Shen, 2005) and
can be reduced by constructing messages using positive and polite language (Aronson & O’Leary,
1983; Reiter & Samuel, 1980). Finally, prompts work most effectively for those behaviours that are
‘simple, easy, effortless and repetitive’ to perform (Frederiks et al., 2015: 1391), and on those
individuals who already feel motivated to engage in the target behaviour (Schultz, 2013).
The persuasive impact of a visual prompt will not only depend on the message and its design, but also
on the recipient’s capacity to attend to and cognitively process the information (Borgstede &
Andersson, 2010). The same authors also suggest that the most important factor for any behavioural
intervention is attracting the attention of the target audience. Most previous studies on prompting
used leaflets or posters as the medium of delivery, yet their effectiveness has been called into
question (cf. Read, 1999). The more permanent nature of a bin sticker may allow for repeated
exposure to the prompt message, thus providing more opportunity than a leaflet for individuals to
notice and cognitively process the message.
Further, it is noteworthy that most experimental research on visual prompts and recycling behaviour
was conducted between 1980 and 2000, at a time when recycling had not yet been established as a
societal norm (Bedford et al., 2010; Thomas & Sharp, 2013). Since this period, there has been a
substantial societal shift in attitudes towards environmental issues in general (Thomas & Sharp, 2013)
and hence further research into the effects of prompting may be warranted. Little is also known about
the longer-term impact and cost effectiveness of behavioural interventions (Abrahamse & Steg, 2013).
With some exceptions (cf. Burn, 1991; Spaccarelli et al., 1990), most existing research on prompting
and recycling behaviour had short-term monitoring periods or did not include longer-term follow-ups
(Schultz et al., 1995). Finally, since most research on behaviour change interventions is academic in
nature, little is known about the actual costs of implementing such strategies in the real world
(Schultz, 2013). Academic studies tend to be small in scale and staff-intensive, often using research
assistants to carefully control implementation. However, when these techniques are implemented at
scale, costs usually increase. It is therefore important for researchers to provide detailed cost
information in addition to reporting results.
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Despite the mixed findings noted above, it seems logical to assume that visual prompts, if properly
designed and implemented, could provide local authorities with one practically feasible and
inexpensive strategy for targeting a large number of households. However, the evidence about their
effectiveness on general recycling behaviour is mixed and, to our knowledge, no academic studies
have investigated their effectiveness for promoting food waste recycling behaviour.
A recent non peer-reviewed research study (WRAP, 2016) sought to assess the effectiveness of
various combinations of the following interventions: a year’s free supply of caddy liners, a leaflet
providing information about how to recycle food waste and a ‘visual prompt’ in the form of a bin
sticker designed to be stuck on the lid of refuse wheelie bins (WRAP, 2016). Working in partnership
with 11 local authorities, a total of 19 pilot studies were carried out between 2013 and 2015, 11 of
which specifically tested the interventions outlined above. The weights of food waste (in tonnes, t)
collected from pre-defined groups of households were monitored before and after the delivery of
each intervention combination. Results were quite variable. The one study that tested the sticker and
liner combination achieved a 13% increase in weight of food waste collected. The mean increase in
weight achieved by the sticker and leaflet combination was slightly higher at 14% (two studies, range
= 4% to 24%). The full ‘package’ of interventions achieved a mean increase of 32% (six studies, range =
18% to 62%). WRAP concluded that the effect on food waste collection rates was amplified when
refuse bin stickers were included. They recommended that local authorities should introduce the full
‘package’ of interventions to have the greatest impact on behaviour, at a cost of £1.12 to £2.10 per
household (WRAP, 2016).
While the addition of the sticker prompt to the other interventions appeared to have an amplifying
effect, it was never tested in isolation. WRAP did not incorporate basic experimental design principles
(e.g., randomisation, replication and control groups) into their study and methodologies for analysing
results also varied between authorities. While sample sizes were often large (up to 15,000 households
in some study areas), the outcome measurement was the weight of food waste collected from
defined groups of households known as ‘collection rounds’. For each pilot study there was an
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insufficient number of collection round weights to statistically analyse results. The collection round
areas chosen to receive the interventions were also not selected at random and no control groups
were included for comparison against a ‘do nothing’ scenario on the part of the local authority.
Further, results from one authority could not always be directly compared with those from another
authority, as there were subtle, yet discernable, differences in the way in which the tonnage data
were recorded and analysed. As a result, WRAP’s findings cannot be generalised to other populations,
as these issues did not allow for a statistically robust set of conclusions to be produced about the
relative effectiveness of each intervention combination.
Separate food waste collection services were first trialled in England between 2006 and 2008
(Bridgwater & Parfitt, 2009). It is therefore surprising that so few academic studies have examined the
effectiveness of policy interventions for promoting this behaviour. A review of the literature found
just three studies that have examined the impact of informational strategies on food waste
separation behaviour. Bernstad et al. (2013) examined the effect of delivering oral information via a
door-stepping campaign. By weighing the separately collected food waste pre and post intervention,
they found that the difference in average weights collected by Swedish households in the control and
treatment groups was not statistically significant. Using the same outcome measures, Bernstad (2014)
assessed the effectiveness of written information (a leaflet about how to recycle and why it is
important) on a different group of Swedish households and found that this type of written
information also did not significantly increase the weight of food waste collected. The third study, a
Randomised Control Trial (RCT) conducted in Manchester, England (Nomura et al., 2011), showed that
feedback comparing a household’s food waste recycling behaviour with other households in the same
street resulted in a statistically significant increase in participation of 2.8% compared with a control
group.
In light of the above, the aim of the present study was to investigate the real world effectiveness,
practical feasibility and cost-efficiency of using sticker prompts to encourage households to recycle
their food waste. Since the true test of any behaviour change intervention lies in its ability to elicit a
change in behaviour, the first objective of the field experiment was to test the effectiveness of the
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visual prompt (a bin sticker) amongst households in two local authorities in Surrey, England. It was
hypothesised that, compared with the baseline period, significantly higher weights of food waste
would be collected in the treatment group during the experimental period than would be collected in
the control group (H1). A secondary objective was to assess the extent to which the impact of the
intervention persisted in the longer term. It was hypothesised that significantly higher weights of food
waste would be collected in the treatment group during each post-intervention period (short,
medium and long term) than in the control group (H2). The final objective of the study was to conduct
a cost-benefit analysis to estimate the payback period for this intervention.
2. Materials and methods
2.1 Participants & setting
Surrey is a county located in the South East of England that operates across 642 square miles and has
a population of over 1.1 million people (ONS, 2011). In Surrey there are a total of 11 district and
borough councils, each acting as Waste Collection Authorities (WCAs), responsible for the collection
of refuse and recycling. Two of these WCAs participated in this study. WCA1 and WCA2 are
responsible for the collection of waste and recycling from 33,538 households and 30,746 households
respectively (Ntotal = 64,284 households).
With regards to key demographics, these areas are broadly representative of England for the most
part. For gender, WCA2 exactly mirrors the national picture, while the split in WCA1 is broadly
equivalent to the national average (England) (ONS, 2011). A greater percentage of the population in
both WCAs are classed as ‘economically active’ (74-75% vs 70% in England) (ONS, 2011). However, of
the population of citizens who are classed as ‘economically inactive’, a higher than average proportion
of the population in both WCAs is retired than there is at the national level (between 6 – 9 % points
higher). This indicates that the older generation are marginally overrepresented in these regions
(ONS, 2011). Indeed, this is reflected in the census age spread data that shows that, compared with
England as a whole, a lower proportion of the population in both WCAs are aged 18-44, while a higher
proportion are aged 45 or over (ONS, 2011). The key area where the WCAs diverge from the national
picture is with respect to education levels. In both areas there is a lower than average (22% in
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England) proportion of residents (16% in WCA1, 20% in WCA2) that have no qualifications. However,
WCA1 has a much higher level of residents that have a qualification at Level 4 or above (36%) than in
WCA2 (26%) or nationally (27%) (ONS, 2011).
Households in WCA1 and WCA2 were issued with food waste recycling caddies in 2009 and 2011,
respectively. However, the recycling rate for food waste in these areas was just 29% (WCA1) and 23%
(WCA2) in 2013/14 (SCC, 2016). Households are grouped together into smaller geographic areas
known as collection rounds, which are residential areas that are serviced by the same collection
vehicle on the same day each week. WCA1 has a total of 20 collection round areas as it operates four
vehicles across five working days, while WCA2 collects from 35 collection rounds as it operates seven
vehicles each weekday. Following collection, the food waste is then taken to a waste transfer station
(WTF) for weighing, before being transported to the AD facility. As the unit of statistical analysis was
the waste collection round rather than the individual household, no background demographic data on
the sample of participants could be obtained. This is due to collection round areas not sharing the
same geographical boundaries that are used for the Census.
2.2 Design
The field study sought to establish whether a visual prompt (the independent variable) would
successfully increase the weight of food waste collected separately for recycling (the dependent
variable). A randomised pre-test/post-test control group design was employed, and the unit of
randomisation was the waste collection round. Table 1 outlines the total number of rounds and
households within each group. The 55 waste collection rounds were randomly assigned to either the
treatment (N=29) or control (N = 26) group using the random number function in Microsoft Excel.
Table 1.
Round information.Groupa Rounds HouseholdsControl 26 30,568Treatment 29 33,716
a Period length for Baseline period (up to 15 weeks) and for Experimental period (up to 16 weeks)
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The study ran for 30 consecutive weeks from the beginning of April 2015 to the end of October 2015.
The baseline period included the 13 weeks immediately prior to the delivery of the treatment and the
week(s) when the treatment (sticker prompt) was delivered (Nbaseline = 14 or 15 weeks, depending on
the authority). The baseline period was inclusive of the treatment delivery week(s) because the
impact of the treatment on resident behaviour could not be recorded until after residents had seen
the sticker and thus had the opportunity to amend their behaviour accordingly. The experimental
period therefore commenced the week following the delivery of the treatment (Nexperimental = 15 or 16
weeks, depending on the authority). Weights for the 26 control group rounds were monitored in the
same way as the 29 rounds in the treatment group.
2.3 Materials
The visual prompt used in this study was a sticker that was designed to encourage and remind
households to use the food waste recycling service, thereby discouraging food waste entry into refuse
bins. Previous research (cf. Aronson & O’Leary, 1983; Jae et al., 2008; Roberts et al., 2009; Sussman et
al., 2012; van Meurs & Aristoff, 2009) on design aspects known to improve the effectiveness of visual
prompts was taken into account during the design process. Stakeholder input was also sought during
a period of consultation with recycling officers and elected council officials from both local authorities.
The sticker design (Figure 1) was produced by professional designers and printed onto durable,
waterproof stickers. The stickers were A5 (148 x 210 mm) in size in order to fit neatly onto the raised
section of refuse wheelie bin lids.
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Figure 1: Sticker prompt design
The primary message, printed in large white bold capitals on a green background, read: “NO FOOD
WASTE PLEASE”. The secondary message, printed below in black lower case lettering on a white
background, prompted individuals to “Remember to use your food recycling caddy”. A picture of a
green outdoor food waste caddy was printed on the bottom right alongside a short web URL that
could be visited should households want further information or require a replacement caddy. Council
logos were printed in black at the very bottom of the sticker.
2.4 Procedure
For sticker distribution, an experienced waste consultancy was appointed to recruit and train
dedicated delivery staff. The consultancy was provided with collection round maps to plan routes in
advance and thus ensure that households could be stickered using the least amount of staff effort. All
collection rounds in the WCA1 treatment group were stickered during week 14 as this is when refuse
was collected. In WCA2, refuse is collected from half of the area one week and from the other half
during the following week. Therefore, some of the treatment group rounds for WCA2 were stickered
during week 14 and others during week 15.
If residents had queries or concerns about the stickers and approached sticker staff, they were
provided with an email address and contact number for the researcher. By deducting the number of
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stickers remaining at the end of each day from the total number of households in each round, it was
possible to estimate the total coverage rate. In total, 98% of treatment group households were
stickered during the exercise period.
2.5 Outcome measurement
To measure and compare changes in food waste recycling behaviour between the control and
treatment groups, weights (in tonnes, t) of separately collected food waste were monitored before
(baseline period) and after (experimental period) the delivery of the stickers. This information was
extracted from an established process that takes place before food waste gets transferred to the AD
facility. Each day, collection vehicles are weighed to determine the daily net weight of food collected
within each collection round. This information is then collated by a third party contractor and
provided to the local authority in spreadsheet format at the end of each month.
The researcher closely examined these data and any missing or unusually high/low weight values
were queried directly with the relevant WCA. If the WCA could not provide missing weight
information, or if their investigation could not provide a reasonable explanation for why a weight
value was unusually high or low, then the weight for that daily round was marked as ‘missing’.
The total weight of food waste collected from each of the 55 collection rounds was recorded for each
week within the baseline and experimental periods. Mean recycling weights for each collection round
were then calculated for each period, with missing weight values discounted from the calculation.
This provided a robust measurement of food waste recycling behaviour within each collection round
for each period. Food waste recycling behaviour for the control group was measured by calculating
the mean weight for the 26 randomly assigned collection rounds for both the baseline and
experimental period. Treatment group weights for both periods were calculated in the same way.
2.6 Data analysis
The assumption of multivariate normality (Tabachnick & Fidell, 2007) was assessed graphically and by
examining skewness and kurtosis statistics. As these both showed the data were normally distributed,
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a parametric test was selected. A repeated measures ANOVA including the independent variable
experimental condition (2 levels: treatment vs. control) as the between subjects factor and the within-
subjects factor time (2 levels: baseline vs. experimental period) was initially conducted to determine
whether the sticker intervention had an effect by comparing the mean weight of food waste collected
during the experimental period with that captured during the baseline period (H1).
In order to test H2, another repeated measures ANOVA was conducted. This time, the mean weight
for each group at baseline was compared with the mean weight for each of the following post-
intervention time periods: short term (the 5 week period immediately following the intervention),
medium term (6 - 10 weeks post-intervention) and long term (11 - 16 weeks post-intervention).
3. Results
The first objective was to test whether providing households with a sticker prompt would significantly
increase the capture of food waste for recycling. It was hypothesised that, compared with the
baseline period, significantly higher weights of food waste would be captured in the treatment group
during the experimental period than in the control group (H1). Table 2 shows that there was no
change in weight of food waste captured for the control group between time periods. However, the
mean weight of food waste collected in the experimental group increased by 20% from 1.23 (SD =
0.35) to 1.49 (SD = 0.37) tonnes, and this increase was statistically significant (t (28) = -10.98, p =
0.00).
Table 2.
Mean weights (tonnes) for each condition and time period.Condition Baselinea Experimentala Difference (%)1. Control 1.24 (0.36)b 1.24 (0.36) -0.00912. Treatment 1.23 (0.35) 1.49 (0.37) 20.7426
a Weight values are weekly mean tonnages captured for all collection rounds in each condition over the baseline and experimental period respectively. b Figures in brackets are standard deviations.
A repeated measures ANOVA showed a significant main effect of time, F(1,53) = 74.15, p < 0.001, and
a significant interaction effect between condition and time, F(1,53) = 74.28, p < 0.001. Figure 2 shows
14
that there was very little difference between experimental groups with respect to mean weights at
baseline, and highlights the significant difference between mean weights during the experimental
period. It can therefore be concluded that, compared with the baseline period, significantly higher
weights of food waste were collected in the treatment group during the experimental period than in
the control group.
Baseline Experimental1.22
1.27
1.32
1.37
1.42
1.47
1.52
ControlTreatment
Time
Mea
n fo
od w
ate
capt
ured
(t)
Figure 2: Mean weights of food waste for both groups pre and post intervention
The second objective was to determine whether this effect was sustained over time. It was
hypothesised that significantly higher mean weights would be collected in the treatment group during
each post-intervention period (short, medium and long term) than would be collected in the control
group (H2). Table 3 shows that the mean weight of food waste captured at baseline (M=1.24, SD =
0.36) in the control group fluctuated across each follow-up period: short term (M = 1.20, SD = 0.34),
medium term (M = 1.28, SD = 0.41), and long term (M = 1.25, SD = 0.38), whereas these weights
clearly increased across time in the treatment group: baseline (M = 1.23, SD = 0.35), short term (M =
1.42, SD = 0.34), medium term (M = 1.74, SD = 0.44) and long term (M = 1.59, SD = 0.49).
Mauchly’s test indicated that the assumption of sphericity had been violated, X2 (5) = 49.99, p = 0.00,
therefore the degrees of freedom were corrected using Greenhouse-Geisser estimates of sphericity (Ɛ
= 0.701). The results show that there was a significant main effect of time, F(2.10, 111.47) = 11.49, p <
0.001, and a significant interaction effect between condition and time, F(2.10, 111.47) = 10.63, p <
0.001.
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Table 3.
Mean weights (tonnes) for each condition and time period.Condition Baselinea Short Medium Long1. Control 1.24 (0.36) b 1.20 (0.34) 1.28 (0.41) 1.25 (0.38)2. Treatment 1.23 (0.35) 1.42 (0.34) 1.47 (0.44) 1.59 (0.49)
a Weight values are weekly mean tonnages captured for all collection rounds in each condition over each of the time periods b Figures in brackets are standard deviations.
Figure 3 highlights the significant differences between mean weights in the short, medium and long
term. It can therefore be concluded that, compared with the baseline period, significantly higher
weights of food waste were captured in the treatment group during each phase of the experimental
period than in the control group. It is perhaps also interesting to note that not only did these
increases persist into the longer term, they also increased over time.
Baseline Short Medium Long1.2
1.25
1.3
1.35
1.4
1.45
1.5
1.55
1.6
ControlTreatment
Time
Mea
n fo
od w
aste
capt
ured
(t)
Figure 3: Mean weights of food waste for both groups by time period
3.1 Cost-benefit analysis
It was important that the policy intervention under evaluation was affordable as well as effective. The
total cost for printing and distributing (i.e., the cost of the delivery staff and their recruitment) the
sticker prompt was £11,702, or £0.35 per household. This cost does not include the cost of sticker
design or the cost of analysing results as these were both completed in-house by existing staff
members. Instead these were recorded as 'in-kind' contributions as such project work typically falls
16
within the remit of local authority staff members and, as such, is rarely factored in as a financial
project cost.
The savings were calculated based on the difference in costs between general waste disposal at
landfill and separate food waste treatment. The cost of disposal varies between local authorities, as
these are dependent on the proportion of waste that gets sent to Energy from Waste facilities (EfW)
vs. landfill, and on the local gate fees that are charged. At the time of writing, the cost per tonne for
AD in the UK (£18 to £38) was much lower than the cost per tonne for general waste disposal (£70 to
£109) (Lets Recycle, 2016). So, for every tonne of food waste that gets diverted away from the general
waste stream, the local authority stands to make a substantial saving.
During the baseline period for this study, the mean weight of food waste collected from the 29
treatment group rounds each week was 1.23 tonnes per round. This increased to 1.49 tonnes during
the experimental period, a difference of 0.26 tonnes. By extrapolating this increase across all
treatment group collection rounds and assuming the increase was maintained over time, using actual
costs of disposal and AD for Surrey in 2014/15 (SCC, 2016) the payback period for this study was
calculated to be just 23 weeks. This, of course, would increase if a drop-off in recycling participation
did occur. Also, an experienced waste consultancy was recruited to ensure that the sticker
distribution was carried out to the highest possible standard during the experimental study. In
practice, stickers could be distributed by in-house staff members (e.g., existing collection crews,
recycling officers) or by agency staff, which could marginally reduce the cost of the intervention. It
may also be possible to achieve economies of scale for the printing and distribution of a larger scale
rollout of this intervention.
In addition to the financial benefits already outlined, there were a few unmeasured benefits
associated with this study. There was a marked increase in requests for new food waste recycling
caddies. The diversion of food waste to recycling would have reduced greenhouse gas emissions and
other associated environmental impacts (Bekker et al., 2010; Nomura et al., 2011).
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4. Discussion
The aim of the present study was to investigate the real world effectiveness, practical feasibility and
cost-efficiency of using stickers as a visual prompt to encourage food waste recycling behaviour. It
was predicted that significantly higher weights of food waste would be captured in the treatment
group than in the control group (H1), and that the impact of the intervention would be sustained over
time (H2).
This study adds to the existing body of literature by supporting the theory that visual prompts, in this
case a simple and practically feasible bin sticker, can be an effective and affordable method of
improving food waste recycling rates, producing effects that appear to persist for up to four months.
These results appear to conflict with previous research that has suggested that, in isolation, visual
prompts are only modestly effective for encouraging recycling behaviour (e.g., Spacarelli et al., 1990).
There are several potential explanations for the impact of this intervention. It is possible that the
success of the intervention could be attributed to the longer-term exposure proffered by the semi-
permanent sticker, compared with leaflets or posters that might only be read once before they are
discarded. Stickers were affixed to the top of refuse bin lids, therefore increasing the opportunities for
them to be noticed and the message to be internalised. As food waste recycling is a relatively new
service, many individuals may be aware of the food waste recycling bins but are still in the habit of
automatically putting food waste into the refuse bin together with other waste. For such individuals,
the prompt may serve as a persistent reminder that they should be separating out their food waste.
Stickers therefore appear to be an effective medium for capturing the attention of a target audience,
as they allow for information to be processed sufficiently (Bernstad, 2014).
It is also possible that the indiscriminate distribution of stickers to all households could be partially
responsible for the effects achieved. By visibly placing stickers on all refuse bins, the local authority
may have signalled to households that food waste recycling was the socially approved behaviour (the
injunctive norm) (cf. Cialdini et al., 1990). Further, as more households began to use their caddies in
response to the sticker, the behaviour may have become more visible to non-recycling households. It
18
is also therefore possible that the descriptive (group) norm for food waste recycling changed in some
areas and motivated a further group of households to start using their own food waste caddies. In this
way, the prompt may have served as a guide to behaviour for some households (Schultz et al., 2008).
Future research could include a follow-up survey to investigate this further.
Several authors have noted that few pro-environmental behaviour change studies have successfully
examined the longer-term effects of behaviour change interventions (Abrahamse et al., 2005;
DeYoung, 1993; Sussman & Gifford, 2012). In this study, the impact of the sticker prompt was
monitored for up to 16 weeks following the distribution of the sticker. Results showed that
significantly higher mean weights of food waste were collected in the treatment group during each of
the short, medium and longer term post-intervention time periods than were collected in the control
group. Further, the weights continued to increase, rather than stay level, across time.
There could be several possible explanations for this finding. Firstly, the sticker distribution resulted in
additional requests for food waste recycling caddies. There was a lead time of several weeks between
caddy requests and delivery, therefore the effect on behaviour for these households would not have
materialised until the medium to long term monitoring periods. It is also possible that there was a
‘time lag’ between some individuals noticing the sticker on the refuse bin and this translating into a
change in behaviour. In some households a different individual could be responsible for disposing of
refuse into the outdoor bin (where the sticker was displayed) than the one who disposes of food
waste indoors. Therefore it could take time for the message to translate between members of a
household and for a collective decision to change behaviour to be made. Again, future research would
be required to investigate this further, perhaps by monitoring the behaviour of a smaller cohort of
households more closely and/or by conducting a follow up survey. Finally, in their study examining
the effect of signs on ‘lights off’ behaviour in public bathrooms, Sussman & Gifford (2012) found that
pro-environmental behaviour returned to near to baseline levels when the signs were removed, and
increased again when signs were re-introduced. It is therefore possible that the more permanent
nature of the bin sticker accounted for the persistency of results found in this study. More research
19
would be required, however, to establish whether this effect would persist beyond 16 weeks as
residents became habituated to the prompt.
The strength of this field experiment lies both in its ecological validity and its practical implications
(e.g., financial savings and environmental impacts) for local authorities (Cialdini, 2009). As the key
goal of this research was to provide practical insights for local authorities to apply in the real world,
high external validity was a priority. A lab-based study could have been used to test the effectiveness
of the sticker prompt but the results would have had high-internal validity and low external validity
due to the artificial setting and probable use of students as participants (Keizer et al., 2013). By
incorporating both randomisation and a control group in this study, the disadvantages associated with
field experiments (cf. Cialdini, 2009; Keizer et al., 2013) were certainly limited. However, the
generalizability of these results could have been improved had there been the opportunity to
replicate the study elsewhere.
While results of this study were promising, there were also several limitations. This study was
designed to inform decision-making concerning the countywide rollout of a programme of policy
interventions designed to encourage food waste recycling behaviour. Since these decisions were time
sensitive to meet targets (Timlett & Williams, 2011), it was only possible to monitor behaviour for a
total of 16 weeks post intervention. While this monitoring period was similar, and in some cases
longer, than can be found in many similar studies (cf. Steg & Vlek, 2009 for further references), future
research would benefit from continuously monitoring behaviour over a longer time period (e.g., six to
twelve months) to examine if and when effects begin to ‘drop-off’ and to what extent. Without
monitoring the effects that occur during the full term of the payback period, the authors cannot
conclusively determine whether the intervention is in fact truly cost effective. Furthermore, although
it was not possible to access such data for the purposes of this study, future research should compare
the weight data for the study with data from the same time period during the previous year, to isolate
any seasonal effects.
20
The design was limited by the spatial level at which the participating local authorities’ could provide
accurate weight data to measure outcomes. For financial and logistical reasons, it was not possible to
measure behaviour at the individual household level (e.g., weigh individual caddies). This meant that
it was not possible to apportion responsibility for the observed increases in weights to: (1) current
food waste recyclers who were reminded to use their caddy more often or more effectively, thereby
improving their performance; or (2) non-recyclers who were persuaded to start using their caddy for
the first time. Measuring the weight of individual food waste caddies requires substantial human and
financial resources, as a dedicated team of research assistants must manually weigh each caddy prior
to collection. A review of the literature found no previous experimental research studies that used
measurements of individual households food waste caddies as an outcome measure. Future research
should explore the logistical and financial viability of this monitoring technique, to be able to
determine the relative responses of each group of individuals to the prompt.
Finally, a manipulation check was also not included, meaning it was not possible to be certain about
how many of the residents who received a sticker actually noticed its presence. Future research could
include a short follow up survey or door-stepping exercise to evaluate residents’ perceptions of the
sticker and the extent to which it was actually viewed and processed.
5. Conclusions
This experimental study tested a simple behavioural intervention in a residential area in South East
England, which would be practically feasible for local authorities to implement at scale. The study
demonstrated that visual prompts, in the form of a green refuse bin sticker, significantly increased the
capture of food waste for recycling and the effect was sustained in the longer term. Additionally, at
£0.35 per household, this intervention appears to be a cost-efficient method for increasing recycling
rates.
While the environmental impact of recycling food waste might be low in comparison to some types of
pro-environmental behaviour, the payout could be high if many individuals are encouraged to engage
(Sussman & Gifford, 2012). Similarly, while visual prompts may not be as effective as other types of
21
behaviour change intervention (Steg & Vlek, 2009), they are one of the simplest and most cost-
efficient to introduce and could represent a ‘quick win’ for local authority waste managers looking to
achieve change with limited budgets. Visual prompts will not change the minds and behaviour of all
individuals, and the individual impact of each person’s behavioural change may be small. However,
the aggregate impact of this intervention, if introduced at scale, could be large.
Acknowledgements
This study was funded by the Engineering and Physical Sciences Research Council (EPSRC) (Student
grant number EP/G037612/1), and Surrey County Council.
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