Data assumptions: 2 sets of data with symmetrical distribution
confidence interval for the difference in the means between 2 sets of data =
= ABS((mean first group) - (mean second group)) +/- (factor)
factor =
= (one-sided value of Student's t-distribution) * (pooled standard deviation) * (((1 / (number in first set)) + (1 / (number in second set))) ^ (0.5))
degrees of freedom =
= (number in first set) + (number in second set) - 2
pooled standard deviation =
= ((A + B) / (degrees of freedom)) ^ (0.5)
A = ((number in first set) - 1) * ((standard deviation of first set) ^ 2)
B = ((number in second set) - 1) * ((standard deviation of second set) ^ 2)
where:
• for a 95% confidence interval, the one-tailed value is for 2.5% (F 0.975, t 0.025)
• as the number of values increases, the closer the one-tailed value for t=0.025 approaches 1.96; at 120 degrees of freedom it is 1.98
