Factors Affecting Choice of Statistical Test Inferential Analysis Factors Affecting Choice of Statistical Test

You are a psychologist. You are already pretty sure that alcohol affects long term memory but you want to find out if alcohol affects your short term memory as well. In pairs, discuss how you could test this in a lab. ? + =

You decide to use an independent measures design to test the effect of alcohol on STM. You split 24 participants into two equal groups: Experimental group: 3 pints of beer Control group: 0 pints of beer. You present lists of digits to the participants and you record the maximum number of digits each participant can recall in one go.

How could you analyse this data? You would generate interval/ratio data (test scores) Test score of ‘alcohol’ group. Test score of ‘no alcohol’ group. 7 5 8 4 6 9 How could you analyse this data?

What could we conclude from the difference in mean score? Results Table to show the number of digits recalled by each group. What could we conclude from the difference in mean score? Mean Range Alcohol 5.75 4 No alcohol 7.0

Inferential Analysis The results appear to show that alcohol impairs STM but: Are the results of your research significant? Ie could they have been a fluke/down to chance? Find out by conducting a statistical test eg Mann Whitney U test Wilcoxon signed ranks test Chi-squared test Spearman’s rank Related or unrelated t test

Lesson Objectives To know why we conduct a statistical test. To know how to select the correct statistical test to complete analysis of results.

Which test do I use? That depends on 3 factors: Whether you’re testing for a difference or a correlation. The design of the investigation (independent measures/repeated measures/matched pairs) What type of data you have (nominal or interval / ordinal) Data that shows the number of things in a particular category eg the number of British people who are underweight and not underweight compared to Chinese people. Data that is ranked in order Data that is continuous eg height, weight or test score.

When Testing for a Difference When Testing for a Correlation Data Type Independent Measures/ Matched Pairs Design Repeated Measures Design ___ Nominal Chi-squared (x ²) Binominal sign test Ordinal Mann Whitney U test Wilcoxon Signed Ranks Test Spearman’s Rank Interval Unrelated t test Related t test Pearson’s r

Task In pairs, decide which statistical test we need to use to find out if there is a significant difference between the participants who drank alcohol and the participants who did not. Be ready to explain your choice. You would need … An unrelated t test You’re testing for a difference, not a correlation. You used an independent measures design. You had interval/ratio data (test scores). Don’t forget the criteria! Difference or a correlation? Which experimental design? What type of data?

Task In pairs, decide which inferential statistics test you would use for the data sets in front of you. When you have finished, join with another pair to compare and discuss your answers.

To know why we conduct a statistical test. Lesson Aims To know why we conduct a statistical test. To know how to select the correct statistical test to complete analysis of results. Have you got the right answers?