# Math Help - 90% and 95 % intervals

1. ## 90% and 95 % intervals

This is my question in the math book.
construct the 90% and the 95% confidence intervals for the population mean.

from a random sample of 36 days in a recent year, the closing stock prices for Tonka toys had a mean of $19.31 and a stardard deviation of$ 2.37

Do I look at the z table? Thank you,

2. Originally Posted by mkcar
This is my question in the math book.
construct the 90% and the 95% confidence intervals for the population mean.

from a random sample of 36 days in a recent year, the closing stock prices for Tonka toys had a mean of $19.31 and a stardard deviation of$ 2.37

Do I look at the z table? Thank you,

If we assume that the closing prices have a normal distribution then we
would be looking at the t table with 35 degrees of freedom (as 35 is a small
sample for these purposes). But without this assumption all we can say
nothing.

Now it might be that you are expected to make some assumtion but without
seeing your notes we don't know if you are supposed to treat 36 as a large
number and so use the z table, or assume the closing prices are normal, but
36 is small and so use the t table.

RonL