-- What do you call a tea party with more than 30 people?
-- A Z party!!!
Joke #123 on http://www.ilstu.edu/~gcramsey/Gallery.html
2*SE is a popular convenient radius to eye 95% CI for t-test. Statisticians take t with df>=30 as z. However, I was incorrect to teach that 1.96*SE could be the precise radius when df>=30.
Guess what is the critical df to use decimally precise 1.96*SE. Larger than expected --