1. ## Standard deviation

Hello
To solve the exercise below, I need the standard deviation =σ I have no clue to get it out of the values below:

A researcher wants to know if the average time in jail for robbery has increased from what it was several years ago when the average sentence was 7 years. He obtains data on 400 more recent robberies and finds an average time served of 7.5 years. A 95% confidence interval for the average time served is:

I have tried the mean of 7 and 7.5 with a n of 2 (standard deviation formula) but that is wrong. I think I get it from the proportion, but then I have no p.
Roger

2. ## Re: Standard deviation

This is not a proportions problem so you really can't do anything without a standard deviation. You'll need more info to solve the problem.

3. ## Re: Standard deviation

I thought that.... lost almost hours on that...
Thank you

4. ## Re: Standard deviation

Also I reread what I wrote and I forgot the "not". Shouldn't change much though - you still need the standard deviation or other information that allows you to figure out the standard deviation.

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