A repeated-measures study comparing two treatments with the sample of n = 4 participants produces mean of M = 18 with SS = 24 for the scores in first treatment, a mean of M = 14 with SS = 18 for the scores in the second treatment, and a mean of M = 4 with SS = 12 for the difference scores. What is the estimated standard error for the sample mean difference?
a. 4
b. 3
c. 2
d. 1