Which Test to Use? Wed, March 31 st

Intro to t-tests & chi-square wNext few weeks, will discuss 3 different types of t-tests & chi-square wHow to decide when to use each test: –Use Decision tree link for today –Consider design of the study: –Examining differences or relationships? –# samples or groups (1 or 2)? –If more than 1 group, were they matched? –If only 1 group, how many scores from subjects?

Decision Tree wWe have already covered correlation & regression, chi-sq is a similar option: –Use corr or chi-sq when interested in whether there is a relationship betw 2 variables –Correlation – relationship betw two interval/ratio variables · is age related to hours of sleep per night?

(cont.) –Chi-square – relationship betw two nominal/ordinal variables · Is gender related to preference for Kerry v Bush? (Lab 24, 4/19)

T-test v Z-test wWe’ve also already covered the z test –Use to compare a sample & pop mean when pop standard dev (  ) is known wZ test is similar to a 1-sample t-test –1-sample t test compares a sample & pop mean, but pop standard dev (  ) is unknown –Will cover 1-sample t in Lab 20 (Apr 5 th )

Types of T-tests wBesides 1-sample t-test, there are 2 other types: –Independent Samples t-test: use when comparing sample means of 2 unrelated (indep) samples –Related Samples t-test: 2 options · Repeated Measures: compare 1 samples’ means at 2 different times · Matched Pairs – Pairs are created & 1 person is put in 1 st group; 2 nd person put in 2 nd group  compare means of Group 1 and Group 2.

Examples of t-tests wInd Samples: compare men and women’s mean job stress scores (Lab 22, 4/12) wRelated Samples (Repeated Measures): compare mean scores of 50 students at pre- test v. post-test (Lab 21, 4/7) –(note: 1 sample, but measured twice) wRelated Samples (Matched Pairs): Pairs of twins – randomly assign Twin 1 to a group, Twin 2 to the other group & compare group means (Lab 21, 4/7) –(note: 2 groups, but they aren’t independent, they’re related)

Lab 19 wNotice 1 branch of decision tree includes comparisons among more than 2 groups –This is beyond 138 material, but may be an answer to one of the lab scenarios (just indicate ‘beyond 138’!) wAlso indicate for ea. Question, 1 or 2- tailed test? Read the Ha – if specifies a direction  1-tailed; if simply “not equal to…” or no difference  2-tailed wSkip #8