Chapter 10
Module 2For self study: Running Tests in SPSS
Assignment 10.2Assignment 2
In an experiment by a group of students it was examined whether humor has a cathartic effect. The students measured participants’ heart rate before and after watching a sit-com, hypothesizing that rates would drop, and assuming that this would be indicative of catharsis.Let us look at their data set (catharsis.sav). At several moments during the procedure the students measured participants’ heart rate. In the list of variables you will find the results (hf1 through hf7). Using a paired sample t-test, investigate whether the change in heart rate was significant (using hf1, that is, heart rate right after participants were instructed on what was going to happen; and hf7, that is, heart rate after seeing an episode of Friends). Read the output and interpret. (a) Report the number of subjects, mean heart-rate (and Standard Deviation, of course) before and after the treatment, t-value, degrees of freedom and p-value. (b) What does this mean for the main hypothesis of this study?
Now do the same for the control condition: compare the scores on hc1 and hc6. (c) Report the number of subjects, mean heart-rate (and Standard Deviation, of course) before and after the treatment, t-value, degrees of freedom and p-value. (d) What do the results mean?
In case all went well you have the following output on your screen.
| Paired Samples Statistics | |||||
|---|---|---|---|---|---|
| Mean | N | Std. Deviation | Std. Error Mean | ||
| Pair 1 | Heart rate after instruction | 84.0000 | 15 | 10.07118 | 2.60037 |
| Heart rate after screening of Friends | 78.1333 | 15 | 9.14851 | 2.36214 | |
As you can see there were 15 subjects, and their mean heart rate was lower after screening Friends. It dropped from 84 (SD = 10.7) to 78.1 (SD = 9.1). Now let us see whether this drop is significant.
| Paired Samples Test | |||||||||
|---|---|---|---|---|---|---|---|---|---|
| Paired Differences | |||||||||
| Mean | Std. Deviation | Std. Error Mean | 95% Confidence Interval of the Difference | t | df | Sig. (2-taied) | |||
| Lower | Upper | ||||||||
| Pair 1 | Heart rate after instruction – Heart rate after screening of Friends | 5.86667 | 7.12006 | 1.83839 | 1.92371 | 9.80962 | 3.191 | 14 | .007 |
The results show that the difference was indeed significant (t = 3.19, df = 14, p < .007). As to the interpretation in terms of the main hypothesis, we do not have enough information yet. What we need to know is whether the drop in heart rate is due to watching Friends or whether it would also occur in a control group watching a serious television show!
To find out we select condition 2 (the control condition) and run the same test again, now with variables hc1 and hc6. This is what you will find:
| Paired Samples Statistics | |||||
|---|---|---|---|---|---|
| Mean | N | Std. Deviation | Std. Error Mean | ||
| Pair 1 | Heart rate before control | 78.5333 | 15 | 10.42570 | 2.69190 |
| Heart rate after showing | 74.5333 | 15 | 9.72380 | 2.51068 | |
Again we see a drop in participants’ heart rate. But the key question is of course, is it a significant change? Let us look at the statistics for the t-test to find out.
| Paired Samples Test | |||||||||
|---|---|---|---|---|---|---|---|---|---|
| Paired Differences | |||||||||
| Mean | Std. Deviation | Std. Error Mean | 95% Confidence Interval of the Difference | t | df | Sig. (2-tailed) | |||
| Lower | Upper | ||||||||
| Pair 1 | Heart rate before control – Heart rate after showing | 4.00000 | 5.29150 | 1.36626 | 1.06966 | 6.93034 | 2.928 | 14 | .011 |
There was a drop in heart rate in the control group, and it was also significant. Although the students used a small data set, it does seem that on the basis of their data, there is no hard evidence for the catharsis hypothesis.