Hi, I am a graduate student in South Korea. Thank you for posting such a nice lecture. It was very helpful. I have a question, though. When you get the pooled variance with three samples for example, you calculated the pooled variance like, Spool^2 = ((n1-1)*s1^2 + (n2-1) *s2^2 + (n3-1)*s3^2 )/df (where df= n1+n2+n3-3). This. But when you get the Standard Error to get t-value using the pooled variance, how would you do it? my friend told me to do like SE= ((Spool^2) /(n1-1) + (Spool^2)/(n2-1))^(1/2) this. But I do not understand why you do not consider all three samples. I mean, it seems like my friend only took into account 2 samples according to the above equation. I don't know if my question is understandable. I will looking forward to your reply. Thank you.