12. Barker and colleagues (2012) compared 61 parents of children who had a serious mental illness (SMI) to 321 parents of children without such an illness. The researchers examined the parents' reported levels of stress, their levels of a hormone called cortisol (levels of this hormone provide an indication of chronic stress), and their use of several types of medication. The table below shows the results of the study. Focusing on the parents' number of stressors (the first row of the table) and the parents' use of medications for anxiety or depression (the last row in the table), explain these results to a person who knows about the t test for a single sample but is unfamiliar with the t test for independent means.

Descriptive Statistics and Mean-Level Comparisons
SMI Parents Comparison Parents
N = 61 N = 321
Mean SD Mean SD t p
Stress
Number of stressors 0.65 0.56 0.42 0.37 -3.08 0.00
Stress severity sum 1.89 1.63 1.16 1.1 -3.3 0.00
Cortisol (nmol/L)
Wake 17.46 10.72 16.75 7.38 -0.14 0.90
Out of bed 22.83 11.48 24.19 10.59 1.13 0.26
Bed 5.72 8.56 4.22 5.57 -1.86 0.06
Cortisol awakening
response 5.62 9.7 7.48 8.34 1.55 0.12
Decline from out of bed 17.13 11.35 20.11 10.71 1.97 0.05
Medication use (proportion)
Allergy 0.26 0.44 0.16 0.37 -1.71 0.09
Steroid 0.2 0.4 0.12 0.33 -1.32 0.19
Hormone 0.13 0.34 0.14 0.35 0.25 0.80
Anxiety or depression 0.26 0.44 0.12 0.33 -2.36 0.02