Solomon, Secker-Walker, Skelly, and Flynn (1996) Journal of Behavioral Medicine studied smoking behavior in pregnant women. They looked at the women’s determination to quit smoking while pregnant. They interviewed 349 women at their first pre-natal visit, all of who were smokers when they became pregnant, and classified them into four groups.
- PC Precontemplation Smokes and has no plan to quit smoking
- C Contemplation Smokes but is thinking of quitting
- P Preparation Smokes, but has made some effort at quitting
- A Action Has already quit
They wanted to look at the subsequent smoking behavior of these subjects over the course of their pregnancy, but one important consideration is how much these women smoked when they became pregnant. If the groups differ on that variable, that might affect the interpretation of the results. (This is our problem for today.) The data can be found in Solomon.sav.
The answers to this exercise may be found at Answers.
The means and standard deviations of these four groups, in terms of cigarettes/day when they became pregnant, follow.
PC |
C |
P |
A |
|
Mean |
24.8 |
16.6 |
28.8 |
13.7 |
St. Dev. |
13.3 |
5.2 |
12.2 |
8.8 |
nj |
69 |
37 |
153 |
90 |
Notice that this is not really a simple problem. Your sample sizes are grossly unequal, and you have problems with heterogeneity of variance. Not to worry, I tell you how to deal with this in the text using the Games-Howell approach. It is a modification of the S-N-K, though can be applied in the context of most pairwise comparisons by making suitable changes in the critical value of qr.
Notice that this is a real data set, and this is the kind of problem that each of you can expect to face in the future. This isn’t some trumped up example that doesn’t apply to anything important. Using the Games-Howell procedure within SPSS, what can you conclude from these data? Tell me how this test differs from a standard S-N-K or Tukey test, as far as the arithmetic is concerned.
Last revised: 04/10/03