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Ch4 Independent Samples T-test HO

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   51 © 2008 Taylor & Francis Group, LLC 4 t  Test for Independent Groups 4.1 Aim The independent t  test is used for testing the difference between the means of two independent groups. It is particularly useful when the research ques-tion requires the comparison of variables (measured at least at the ordinal  level) obtained from two independent samples.For example:“Do males and females differ in performance on a standardized achieve-ment test?”“What is the effect of drug versus no drug on rats’ maze learning behavior?”“Does the recidivism rate of juvenile offenders who are provided with father figures differ from those without father figures?” 4.2 Checklist of Requirements In any one analysis, there must be:ã Only one independent (grouping) variable (IV) (e.g., subject’s gender)ã Only two levels for that IV (e.g., male, female)ã Only one dependent variable (DV) 4.3 Assumptions ã Independence —The two groups are independent of one another.ã Normality —The dependent variable is normally distributed.ã Homogeneity of variance —That is, the distribution of the depen-dent variable for one of the groups being compared has the same variance as the distribution for the other group being compared.  52  Handbook of Univariate and Multivariate Data Analysis with IBM SPSS © 2008 Taylor & Francis Group, LLC 4.4 Example A researcher wants to investigate whether first-year male and female stu-dents at a university differ in their memory abilities. Ten male students and 10 female students were randomly selected from the first-year enrolment roll to serve as subjects. All 20 subjects were read 30 unrelated words and were then asked to recall as many of the words as possible. The numbers of words correctly recalled by each subject were recorded. 4.4.1 Data Entry Format The data set has been saved under the name EX4.SAV 4.4.2 Testing Assumptions 4.4.2.1 Independence During data collection, ensure that the observations in one group are independent of the observations of the other group. 4.4.2.2 Normality  4.4.2.2.1 Windows Method  1. From the menu bar, click Analyze , then Descriptive Statistics , and then Explore… . The following Explore  window will open. MalesFemales s1 16s1 24s2 14s2 23s3 18s3 26s4 25s4 17s5 17s5 18s6 14s6 20s7 19s7 23s8 21s8 26s9 16s9 24s10 17s10 20 VariablesColumnCode Gender11 =  male, 2 =  femaleWords2Number of words correctly recalled  53t  Test for Independent Groups © 2008 Taylor & Francis Group, LLC  2. Transfer the WORDS  variable to the Dependent List:  field by click-ing this variable (highlight) and then clicking . 3. Click to open the Explore: Statistics  window. Check the Descriptives  field and click to return to the Explore  window.  54  Handbook of Univariate and Multivariate Data Analysis with IBM SPSS © 2008 Taylor & Francis Group, LLC  4. In the Explore  window click to open the Explore: Plots  window. Check the Normality plots with tests  field. Click to return to the Explore  window. 5. When the Explore  window opens, click to complete the analysis. See Table 4.1 for the results.
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