Correction: the convenient radius for 95% confidence interval of t-test

-- 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 --



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