this study, the outcome of interest was the change in womens' responses on the Hamilton Depression Rating Scale between the treatment (antidepressant + oral creatinine supplementation) and control (antidepressant + oral placebo supplementation). Because of randomization, the Hamilton Depression Rating Scale score distributions were nearly identical between the treatment and control groups at baseline (start of the study). As such, the Hamilton scores at the end of the eight-week follow-up period can be compared to assess differences, if any, in the average Hamilton Score change.

The Hamilton Depression Rating Scale is an aggregate scale based on user responses to questions about specific psychological and physical symptoms of depression. Higher scores indicate greater severity of depression. A copy of the Hamilton Depression Rating Scale can be found at http://healthnet.umassmed.edu/mhealth/HAMD.pdf.

Summary statistics about the resulting Hamilton Depression Rating Scale (HDRS) scores at the end of the eight-week clinical trial are as follows:

Group n mean standard deviation Treatment 17 5.4 3.0 Control 22 9.8 3.5

For the "Treatment" group, the estimated standard deviation of the 17 HDRS scores is 3.0. What does this quantity estimate?

a. This quantity estimates the variability of individual HDRS scores among the population of women with major depressive disorders who given the Treatment.

b. This quantity estimates the variabililty of sample mean HDRS scores across all possible random samples of n=17 from the population of women with major depressive disorders who were given the Treatment.

c. This quantiy estimates how reliable the HDRS scores are in measuring depression.