Here are some problems we can work on for lab:
Thirty-two students are randomly selected from a large introductory class and are randomly and evenly assigned to one of two groups. Each participant is given four 15-second exposures to a list of 15 nouns. After 30 minutes, each participant in one group is given a free recall test, whereas each participant in the other group is given a recognition test. The results in terms of the number of words correctly retained are shown here. Compare the groups.
| Recall Test | Recognition Test | ||
| X | f | X | f |
| 4 | 1 | 15 | 6 |
| 2 | 5 | 14 | 1 |
| 1 | 7 | 13 | 4 |
| 0 | 3 | 12 | 2 |
| 10 | 1 | ||
| 8 | 1 | ||
| 6 | 1 | ||
Ten politically active individuals are selected, all of whom label themselves as liberal. Each is attached to a physiograph, and heart rate is recorded while a sequence of 20 slides is projected on a screen at which the participant is looking. Half of the slides are pictures of famous individuals without any expressed political philosophy; the other half are known conservatives. The arrangement of the slides is random. Each participant receives two scores: The first is the average heart rate during exposure to the neutral slides, and the second is the average heart rate during exposure to the conservative slides. The data are shown here. Does reaction to the slides differ?
| Participant | Neutral | Conservative |
| 1 | 65.3 | 71.8 |
| 2 | 75.7 | 73.5 |
| 3 | 85.6 | 99.3 |
| 4 | 73.7 | 81.7 |
| 5 | 69.5 | 75.7 |
| 6 | 68.2 | 73.5 |
| 7 | 70.1 | 79.8 |
| 8 | 72.5 | 70.3 |
| 9 | 71.0 | 85.3 |
| 10 | 83.5 | 107.1 |
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USING SPSS—EXAMPLES AND EXERCISES
The SPSS procedure for computing t tests has options that will compute both the t test for independent samples and the t test for dependent samples.
Example: First, we will use SPSS to work an exercise for the t test for independent samples—specifically, the problem above. The steps are as follows:
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Start SPSS. Data entry is slightly different for this type of problem. Name the first variable group and the second, words. Enter the data for the Recall Test group under words with a 1 in the group column, and enter the data for the Recognition Test group under words but with a 2 in the group column in front of the data. Part of the Data Editor showing the pattern of data entry follows:
- Choose Analyze>Compare Means>Independent Samples T Test.
- Move words into the Test Variables box and group into the Grouping Variable box.
- Click on Define Groups and enter the code values for Group 1 and Group 2. The code values will be 1 and 2 because these are the numbers we entered to identify the groups. ClickContinue>OK. The results should appear in the output Viewer window.
Notes on Reading the Output
- The t value we will use is the one in the first row labeled "equal variances assumed," because this is one of the assumptions we have made. (Levene's Test is a test of the homogeneity of variances assumption.)
- "Sig. (2-tailed)" is the exact p value for the computed t. We reject the null hypothesis if this value is ≤ .05 and conclude that there is a significant difference between the groups.
Example: Now we will use SPSS to work an example of the t test for dependent samples—Problem 6. The steps are as follows:
- Start SPSS, name variables neutral and conserv, and enter the data.
- Analyze>Compare Means>Paired-Samples T Test.
- Move both variables into the Paired Variables box, then click OK. The results should appear in the output Viewer window.
Notes on Reading the Output
- You should find the results reasonably easy to identify and interpret in this output. The critical results are found in the Paired Samples Test box. The results indicate that t(9) = –3.389 andp = .008. As before, SPSS labels the p value as "Sig. (2-tailed)." Because the obtained p value is less than our alpha level of .05, we reject the null hypothesis and conclude that there is a significant difference in the heart rates, which increase when a liberal views the slide of a known conservative.
Exercises Using SPSS
- Use SPSS to work the following problem (data below), assuming the data are from independent groups; that is, assume that 20 participants were randomly assigned to attend or not to attend a speed reading course. At the conclusion of the course, all participants were given an evaluation to measure their reading speed in words per minute. Was the class effective? (Hint: Don’t forget the differences in the data input arrangement between the independent and dependent t tests.) Write a conclusion for this problem.
- Use SPSS to work teh speed reading problem just as stated—as a dependent t test problem. You will have to re-enter your data. Compare your results with those from SPSS Exercise 1. Given the same data, which design is more powerful?
A group of 10 students completed a class to improve reading speed. Their average reading speed, inwords per minute, was measured before and after taking the class. Was the class effective?
Ss Speed Before Speed After A 476 519
B 342 460 C 527 499 D 261 450 E 439 610 F 517 547 G 391 519 H 422 662 I 321 456 J 410 420