In exercises that call for two-sample procedures, you may use either of the two approximations for the degrees of freedom that we have discussed: the value given by your software or the smaller of and . Be sure to state clearly which approximation you have used.
7.51 Trustworthiness and eye color.
Why do we naturally tend to trust some strangers more than others? One group of researchers decided to study the relationship between eye color and trustworthiness.25 In their experiment, the researchers took photographs of 80 students (20 males with brown eyes, 20 males with blue eyes, 20 females with brown eyes, and 20 females with blue eyes), each seated in front of a white background looking directly at the camera with a neutral expression. These photos were cropped so the eyes were horizontal and at the same height in the photo and so the neckline was visible. They then recruited 105 participants to judge the trustworthiness of each student photo. This was done using a 10-point scale, where 1 meant very untrustworthy and 10 very trustworthy. The 80 scores from each participant were then converted to -scores, and the average -score of each photo (across all 105 participants) was used for the analysis. Here is a summary of the results:
| Eye color | |||
|---|---|---|---|
| Brown | 40 | 0.55 | 1.68 |
| Blue | 40 | −0.38 | 1.53 |
Can we conclude from these data that brown-eyed students appear more trustworthy compared with their blue-eyed counterparts? Test the hypothesis that the average scores for the two groups are the same.
7.51
. (c) . The data show that brown-eyed students appear more trustworthy compared with their blue-eyed counterparts.