I want to confirm the following statement If population is given, interval estimate for 90% is mean + 1.65 * sigma If sample is given, interval estimate for 90% is mean + 1.65 * sigma/ sqrt(n) Appreciate any comments on how you use one formula vs other.

If sample size is given, then you will have to go to the t-distribution table and find the reliability factor for n-1 df. Otherwise u can use 1.65 if no sample size is given.

Do a search on this or check it out in the book againâ€¦