"Women" are the participants and they are a constant. Since there are only women in the study (no men) women cannot be a variable since there is no variability. It is helpful to recognize constants because sometimes it is easy to get constants confused with variables. Also, while this tutorial does not develop external validity in a detailed way, constants re very important to recognize so you know how far you can safely generalize the results of a study which has constants. For example, there have been some problems with knowing the helpfulness of and best dosage for prescription drugs for women since most of the testing has been done on men. There have also been problems with generalizing these drug effectiveness studies to the elderly and children. This shows how it is important to recognize the constants in a study. If you looked at Garrity and Degelman (1990) carefully you may have noticed one of the constants in that study was the gender of the the servers; they were all women. It might be that a replication that included men servers would get different results because people might react differently to men and women servers introducing themselves by name.