# ISU PSY 138 - Related Samples t-test (10 pages)

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## Related Samples t-test

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## Related Samples t-test

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Problems/Exams

Pages:
10
School:
Illinois State University
Course:
Psy 138 - Reasoning in Psychology Using Statistics
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Related Samples t test Wed Apr 7th Related Samples t Use when 1 group of subjects is tested more than once e g pre test post test or 2 groups of related or matched subjects are measured once e g twins Uses same basic t formula but focus now on difference scores Differences in pre and post test or difference in each partner in a pair Hypotheses Null hyp Ho indicates no difference or no effect so we ll hypothesize the mean difference in the pop D 0 Ha indicates there will be a difference 2tailed D not 0 or there will be a difference in a specific direction 1 tailed D 0 or D 0 We ll reject Ho if t obs t critical t observed is in critical region Ex What is the effectiveness of a new program to reduce litter Measured aver litter in 4 cities in 01 and 02 City July 2001 July 2002 Fresno 9 2 Merced 10 4 Bakersfld 8 9 Stockton 9 1 Find D difference score for each city and then the average D D bar City July July 2001 2002 D Fresno 9 2 7 Merced 10 4 6 Bakersfld 8 9 1 Stockton 9 1 8 D bar 20 4 5 Next sum up the deviation scores D Then find squared deviation scores for each city D2 add a new column Then sum up the squared deviation scores D2 Here D 20 D2 150 Use the T observed formula T formula for Related Samples Note D always T observed Dbar D 0 SDbar Where SDbar std error sqrt SD2 N and SD2 D2 D 2 N 1 150 20 2 3 83 33 SDbar sqrt 83 33 4 4 56 T obs 5 0 4 56 1 09 Finding T critical Use the t table as before Need alpha level 1 or 2 tailed test Need Df N 1 here N pairs of scores or total participants if repeated measures Ex use 01 1 tailed test expected litter to decrease Df 3 T critical 4 541 T observed 1 09 So fail to reject Ho t obs t critical There is no difference in litter before after new system not effective SPSS GSS98 dataset example Analyze Compare Means Paired Samples t test Pop up window select wife for var1 children for var2 then click arrow to put them in Paired vars box OK cont In output 1st section is Paired Samples Stats look for means for wife and children this

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