The estimated standard error for the sample mean difference is  2.5 .
According to the question
A repeated-measures study comparing two treatments 
n = 4 
MD(mean difference) = 2
SS (sum of square) = 75 
Now, 
error for the sample  
Formula for standard error 

by substituting the value 
 
 
 
 
S² = 25
S = 5 (s is never negative) 
Standard error of the estimate for the sample mean difference 
As
The standard error of the estimate is the estimation of the accuracy of any predictions.
The formula for standard error of the mean difference 
standard error of the mean difference  = 
   
standard error of the mean difference =  
   
standard error of the mean difference = 2.5 
Hence, the estimated standard error for the sample mean difference is  2.5 .
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