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STUDY DESIGNS: case control, cohort and qualitative. Dr TAMSIN Newlove-delgado DOCTORAL RESEARCH FELLOW UNIVERSITY OF EXETER. Aims and objectives. To outline and revise: Causation Case control study design, advantages and disadvantages The odds ratio - PowerPoint PPT Presentation
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DR TAMSIN NEWLOVE-DELGADODOCTORAL RESEARCH FELLOW
UNIVERSITY OF EXETER
STUDY DESIGNS:case control, cohort and
qualitative
Aims and objectives
To outline and revise:Causation
Case control study design, advantages and disadvantages The odds ratio Cohort study design, advantages and disadvantages Relative risk
To summarize some key points about qualitative study design: Use Methods Advantages and disadvantages
Relevant Paper 3 Syllabus
3.1.10. Knows the benefits and weaknesses of different quantitative study designs to address different clinical questions: Case-control Cohort
3.6 Critically appraises cohort and case control studies
Relevant Paper 3 Syllabus
3.4. Qualitative Methods Knows when to apply qualitative research methodologies Additional approaches to sampling in qualitative studies Different approaches to data gathering in qualitative
studies The role of qualitative methodologies in instrument (i.e.
screening, diagnostic, outcome measure) development Methods for validating qualitative data Methods for minimising bias Methods of analyzing data Data saturation
3.6 – Critically appraises qualitative research
Plan of afternoon
1pm-2.30pm – Case control and cohort studies: including coffee break and exam questions
2.30pm – 3.30pm – Qualitative studies
1. Causation and study designs
Example MCQ
Which of the following is not one of the Bradford-Hill criteria?
a)Temporalityb)Biological gradientc)Plausibilityd)Residual confoundinge)Strength of association
http://www.youtube.com/watch?v=fFfWykH05Gw
The Daily Mail List of Things That Give You Cancer
AGE: http://www.dailymail.co.uk/news/arti...st-cancer.htmlAIR POLLUTION: http://www.dailymail.co.uk/health/ar...ld-cancer.htmlAIR TRAVEL: http://www.dailymail.co.uk/health/ar...ncer-risk.html andhttp://www.dailymail.co.uk/travel/ar...nt-fliers.htmlALCOHOL: http://www.dailymail.co.uk/news/arti...ncer-risk.html andhttp://www.dailymail.co.uk/health/ar...-wine-day.htmlALLERGIES: http://www.dailymail.co.uk/health/ar...ncer-risk.html ARTIFICIAL FLAVOURS: http://www.dailymail.co.uk/news/arti...soy-sauce.html ARTIFICIAL LIGHT: http://www.dailymail.co.uk/health/ar...ncer-risk.html ASBESTOS: (as if it wasnt bad enough already) http://www.dailymail.co.uk/news/arti...ng-cancer.html ASPIRIN: http://www.dailymail.co.uk/health/ar...ncer-risk.html BABIES: http://www.dailymail.co.uk/news/arti...st-cancer.html BABY BOTTLES: http://www.dailymail.co.uk/health/ar...fertility.htmlBABY FOOD: http://www.dailymail.co.uk/health/ar...cer-alert.htmlBACON: http://www.dailymail.co.uk/health/ar...cers-grow.html BARBEQUES: http://www.dailymail.co.uk/news/arti...ncer-risk.htmlBEEF: http://www.dailymail.co.uk/news/arti...st-cancer.html BEER: http://www.dailymail.co.uk/health/ar...te-cancer.htmlBEING A BLACK PERSON: http://www.dailymail.co.uk/health/ar...cancer.htmlandhttp://www.dailymail.co.uk/health/ar...ite-women.htmlBEING A WOMAN: http://www.dailymail.co.uk/health/ar...n-smokers.html BEING A MAN: http://www.dailymail.co.uk/health/ar...cer-women.html BEING SOUTHERN: http://www.dailymail.co.uk/health/ar...st-cancer.htmlBISCUITS: http://www.dailymail.co.uk/femail/ar...ods-avoid.html
BRAS: http://www.dailymail.co.uk/news/arti...a-bad-you.html BREAD: http://www.dailymail.co.uk/health/ar...ncer-risk.html
RCT: not always the answer1
UnnecessaryImpractical/UnethicalInappropriate
Prognosis Diagnosis Quality issues And more
Study designs2
Objective Common design
Prevalence Cross-sectional
Incidence Cohort
Cause Cohort, case-control, cross-sectional (in order of reliability)
Prognosis Cohort
Treatment effect Controlled trial
Issues of how, why etc as opposed to what or how much
Qualitative design
Investigating aetiology
Epidemiological studies of aetiology are usually observational not experimental
An observed association may be due to: True cause Reverse causation Chance (random error) Bias (systematic error) Confounding
Investigating aetiology
Assessing causation
Criteria?
Questions of causation
The Bradford-Hill criteria (J Roy Soc Med 1965:58:295-300)
1. Strength of the association.2. Consistency of findings.3. Specificity of the association.4. Temporal sequence of association.5. Biological gradient.6. Biological plausibility.7. Coherence.8. Experiment.
Can you think of examples where this doesn’t work?
2. Case control designs
The Case Control Study: Design
The case control study: design3
Advantages
Efficient for studies of rare diseases and diseases with long latent periods
Cheap, simple, quick (in comparison to cohorts)
Can examine multiple exposures – generate hypotheses
Sometimes the only practical option (e.g. where long latent period between exposure and disease)
Disadvantages
There are many!Can study only one outcomeNotorious for being prone to bias:
Sampling/selection bias – selection of cases and controls
Observation and recall bias
Not good for rare exposuresThe temporal sequence between exposure
and disease may be difficult to determine.As with all studies, confounding
Selecting cases
Need a clear case definition and source Cases selected for a study should be
representative of all cases of the disease in the population.
This is difficult!: many cases not diagnosed or misdiagnosed
A hospital sample in some diseases may be very different from a community sample
Selecting controls
Controls are used to estimate the prevalence of exposure in the population which gave rise to the cases.
The ideal control group would comprise a random sample from the general population that gave rise to the cases.
Controls should meet all the criteria for cases, apart from having the disease itself; but they should have the potential to develop it
Methods of selecting controls
Convenience sampleMatched sampleUsing two or more control groupsUsing population base sample e.g. from
registers
Recruiting more than one control per case may improve the statistical power of the study (up to 4 controls per case)
Selecting controls: matching
Matching – Some studies are matched to select cases/controls who are as similar as possible e.g on age, ethnicity etc
Can be useful in small samples – as we might not have sufficient subjects to adjust for several variables at once.
Difficult/complicated to match on too many factors
Important not to match on basis of risk factor of interest / too many factors – ‘overmatching’ may make the controls unrepresentative and underestimate the true difference
Example of selection bias
• Bias introduced through poor selection of controls
• Case control studies of NSAIDS (exposure) in colorectal cancer3
Case control studies in psychiatry
Suicide a popular subject……. Barraclough, B., Bunch, J., Nelson, B., et al
(1974) A hundred cases of suicide: clinical aspects. British Journal of Psychiatry, 125, 355-373.
More modern examples: Fuller Torrey E, Rawlings R, Yolken RH. The
antecedents ofpsychoses: a case-control study of selected risk factors.
Schizophr Res2000; 46: 17–23.
Case control studies and the odds ratio
Estimates the strength of association between an exposure and an outcome
Does not calculate relative risk as retrospective
Does not give incidence/prevalence – unless all cases in a population are included
The odds ratio is a measure of the odds of exposure in the cases, compared to the odds of exposure in the control group.
OR: 2 by 2 table
Cases Controls Total
Exposed a b A + b
Unexposed c d C + d
Total A + c B +d
OR = (a/c)/(b/d)
Imaginary worked example – Cats and schizophrenia
Schizophrenia Controls
Owned cat as child 80 100
Did not own cat as child
20 300
Total 100 400
Imaginary example– are cats associated with schizophrenia?
Odds of exposure in the cases: 80/20 = 4Odds of exposure in the controls: 100/300 = 0.33Odds ratio: 4/0.33 = 12.12So……the odds of having had a cat as a child in the
group with schizophrenia were over 12 times the odds of having had a cat as a child in the control group –
Or those with schizophrenia were over 12 times more likely to have had a cat as a child….
Why might we get this result?
Feline bias
Selection bias Cases recruited through a charity that runs ‘pet
experiences’ for people with mental illness Controls were a hospital sample recruited from an
allergy clinic Both of these would spuriously increase estimate of
effect
Recall bias Are those with schizophrenia more likely to
remember/report having had a cat – particularly if aware of hypothesis in question
3. Cohort study designs
Cohort study design
Usually prospective; but can be retrospective
A prospective cohort
Prospective and retrospective cohort
Cohort studies may be prospective or retrospective, but both types define the cohort on the basis of exposure, not outcome.
Prospective and retrospective cohorts
Advantages
Can investigate risk factors impossible to study in controlled trials - e.g. smoking or asbestos
Describe incidence and natural historyMultiple outcomes can be measured for any one
exposure.Exposure is measured before the onset of disease
(in prospective cohort studies).Good for measuring rare exposures, for example
among different occupations.Demonstrate direction of causality.Can calculate relative risk
Disadvantages
Expensive, time consumingLoss to follow up can introduce biasNeed a large sample size – especially for less
common outcomesNot good for rare outcomes or long latency
periodsNeed to maintain consistency of follow up
over timeSystematic misclassification of exposure or
outcome status – information bias
Sources of bias in cohort studies
Differential misclassification: can lead to an over- or underestimate of the effect between exposure and outcome.
Losses to follow up : degree to which losses to follow up are related to either exposure or outcome can lead to serious bias in the measurement of effect of exposure and outcome.2
Cohort studies in psychiatry
Andreasson et al: cannabis consumption and development of schizophrenia in a cohort of 45,570 Swedish conscripts4.
Relative risk in cohort studies
Analysis Riskexp = a / (a+c) (divide by total exposed)
Riskunexp = b / (b+d) (divide by total unexposed)
Estimate relative risk = Riskexp / Riskunexp
Indicates increased/decreased risk of disease assoc with exp: RR = 1 – risk is same in exposed and unexposed groups RR > 1 – risk is greater in exposed group RR < 1 – reduction in risk in exposed group Exposed to factor:
Yes No Total
Disease of interest:
Yes a b a+b
No c d c+d
Total a+c b+d N = a+b+c+d
Example: relative risk from Swedish conscript cohort study4
Cannabis exposure
None Low Medium High
Schizophrenia
197 18 10 21
No schizophrenia
41083 2818 692 731
Total 41280 2836 702 752
Exam questions – example EMIs
4. Qualitative studies
Qualitative studies
Answers questions such as: What is X & how does X vary in diff circumstances & why? Not ‘how big is X or how many X’s are there?
Concerned with meanings people attach to their experience & how they make sense of world
Some features of qualitative research
Uses of qualitative research
Preliminary to quantitative research Helps ensure validity of data obtained E.g. interviews to inform a survey Generate theory
To validate quantitative research or provide a diff perspective on same phenomena. Mixed methods
Used independently to uncover processes or practice not amenable to quantitative research
Address the 'gap' between evidence based approaches and practice
Example: Owens et al 20115
Objective To shed light on the difficulties faced by relatives, friends, and colleagues in interpreting signs of suicidality and deciding whether and how to intervene.
Design Qualitative study of completed suicides, based on in-depth interviews with multiple informants.
Setting London, southwest England, and south Wales. Participants 31 lay informants (one to five for each case),
including parents, partners, siblings, friends, and colleagues of 14 cases of suicide in which the deceased was aged 18-34 and was not in contact with secondary mental health services
Quotes5
Friend: “He’s a bloke, isn’t he? We don’t do emotion.”
Sibling: “I’d try and get things out of him about how he felt, especially when he’d had a few drinks, but he never ever opened up . . . He was a typical bloke in that respect . . . So I never really pushed it. I didn’t want to make him feel uncomfortable.”
Interpretation/themes arising
‘‘Informants also mentioned the strong social pressures, especially among young men, to hide distress and keep up a pretence of coping. The social contexts in which young men meet, such as pubs and clubs, may not be conducive to “troubles telling,”26 and lack of emotional literacy may mean that they do not have the vocabulary or skills to do so.
These quotations betray a tacit acceptance of this situation on the part of significant others, and even an expectation that boys will not talk about emotions, which may have served to reinforce inhibitions and close down opportunities to confide.’’(Owens et al 2011)
Qualitative vs. quantitative
Quantitative Qualitative
Type of reasoning Deduction Induction
Objectivity Subjectivity
Causation Meaning
Type of question Pre-specified Open-ended
Outcome orientated Process orientated
Type of analysis Numerical estimation Narrative description
Example MCQs
In qualitative research, which of the following refers to modifying the research methods and hypothesis as and while one conducts the research?
A. TriangulationB. Iterative approachC. Theoretical samplingD. Content analysisE. Deductive approach
Example MCQs
In qualitative research, which of the following terms does not refer to a method of selecting a sample:
A.SnowballingB.PurposiveC.ConvenienceD.Framework approachE.Theoretical sampling
Example MCQs
In qualitative research, the inclusion of which of the following may be an indicator of the quality of a study? Sensitivity analysis Reflexivity Bootstrapping Snowballing Blinding
Broad methodologies
Grounded Theory (Glaser and Strauss) Social and psychological processes – may generate
theory/models Key concepts: theoretical sampling and constant
comparison
Phenomenology Lived experience rather than theory generation
Ethnography Learning about a culture from the people that live in it
Data collection
InterviewsFocus groupsObservational approaches
Participant observation Ethnographic Discourse/conversational analysis
Documentary analysis
Qualitative vs quantitative approaches to sampling
Often a smaller sample size – but rich in detail
Quantitative research uses probability sampling
Qualitative research uses non-probability sampling not representative samples findings cannot be generalised to the whole study
population from which the sample was taken. the people in the study population do not each have
an equal chance of being selected.
Concepts in qualitative sampling
Some sampling approaches
Process of analysing qualitative data
Data preparation Nature and scale of qualitative data Transcription
Relationship between data and analysis Transcripts provide descriptive record Analytical process begins during data collection as data
already gathered are analysed and feed into/shape ongoing data collection
QUANT QUAL
CollectAnalyse
CollectAnalyse
Analysing qualitative data
Goal To develop analytic categories to describe & explain May be
derived inductively (from the data) or deductively (predefined themes drawn from schedule & research Qs)
Broad approaches Thematic analysis Grounded theory Framework approach
Initial steps Manage & make sense of data Reading & re-reading to identify initial set of themes Coding, label themes, use their language Organising, grouping & refining of themes/categories
Specific methods of analysis
Quality in qualitative research
Summary – Qualitative research
Some key words/concepts to be familiar with - see syllabus
Cannot apply same rigid concepts re appraisal etc as for quantitative research
However, there are ways of considering the quality of a piece of qualitative research
Also a CASP critical appraisal tool available: http://www.casp-uk.net/
References
1. Greenhalgh T. How to Read a Paper2. Mann CJ. Observational Research methods: Research
design 2: cohort, cross-sectional and case-control studies. Emerg Med 2003 20: 54-60
3. Schulz KF and Grimes DA. Case Control Studies: research in reverse. The Lancet doi:10.1016/S0140-6736(02)07605-5
4. Andreasson S., Engstrom A., Allebeck P., Rydberg U. Cannabis and schizophrenia. A longitudinal study of Swedish conscripts (1987) Lancet, 2 (8574), pp. 1483-1485+1486.
5. Owens, C., Owen, G., Belam, J., Lloyd, K., Rapport, F., Donovan, J., & Lambert, H. (2011). Recognising and responding to suicidal crisis within family and social networks: qualitative study. BMJ, 343(oct18 1), d5801–d5801. doi:10.1136/bmj.d5801