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Tuesday 9-11 (group 2), Tuesday 11-1 (group 3)

Tom Mitchell t.mitchell@abdn.ac.uk room T15

mailto:t.mitchell@abdn.ac.ukmailto:t.mitchell@abdn.ac.uk

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MCQs 7.2 & 7.3

What is a t-test actually comparing??

Data-analysis in groups

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Is there a genuine difference between our 2 group means?

In other words: is the difference between our 2 group

means statistically significant?

Mean X Mean Y?

Need to take Overall Variability in the data into account

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Group As mean IQ: 100

Group Bs mean IQ: 105

Scenario 1

range of scores for Group A = 99-101, range for

Group B = 104-106.

5-point difference = genuine difference

Scenario 2

range of scores for Group A = 65-170, range

for Group B = 67-171. 5-point difference = no genuine difference

Is this 5-point difference

statistically significant?

T-test: Is the difference between groupsmeaningful relative to the total variability?

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21

21

Group

Group2

Group

Group1

GroupGroup

N

variance

N

variance

)( 21

21

GroupGroup

GroupGroup

SE

T-value =

=

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In groups: calculate your observed t-value from your owngroup data (some of you may find a significant differencesome of you may not).

Produce a bar graph which clearlyillustrates the data.(remember use full scale, error bars and correct labellingof x and y axis.

FIGURE reference should tell the reader what measure ofcentral tendency has been used (e.g. mean), and what isshown by the error bars (st.error or st deviation).

Did you find a significant difference between your group means?

In other words: did you find a significant effect of Stimulus Type?

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Standard deviation: distribution of scores inyour sample4 friends are on a budget night out.A = 2 drinks, B = 3 drinks, C = 3 drinks, D = 4 drinksMean: 12/4 = 3 drinks, SD: 0.4

Standard error: indicates how the sample mean varies if you take manysamples across the population

How likely is it, across the country, to find a group of 4 people who have anaverage of 3 drinks?

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words pictures0

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berofitemsremembered

Significant PSE

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words pictures0

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berofitemsremembered

No Sig diff between groups

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words pictures

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Opposite effect words are better remembered.

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Using the worksheet I have lovingly prepared availableon myAberdeen work in pairs at computer to conductyour t test and produce a graph of your group data(therefore do not try to work with someone from adifferent research groupthis wont work very well)

You will therefore have produced the content of yourresults section in class, you will need to structure thisinto your report.

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Back into your research group to discuss what your data shows.Think about factors which may have contributed to a significant

effect (or indeed an absence of any effect in your data)

Important factors:

Speed of presentation; words encoded instantly, pictures

require interpretation

Ambiguity of picture; what is it???

Only concreteness controlled for (syllables, letters, frequency,imagery)

Participants aware of hypothesis; trying to visualise words?

Was the filler task demanding enough?

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Continue writing up Introduction, methods, results

Next weeks class will focus on discussion: Think aboutreasons why you did/did not find significant effects in yourgroups and come prepared to discuss these in class.

Think about improvements to the methodology, try to move

away from sample population as an issue (I am afraid mostresearch in Psychology is conducted with undergraduatepopulations).-how could you improve the experimental set-up to look deeper into the effect?

Next week: GPP 1