# Thread: Confidence interval by hand

1. ## Confidence interval by hand

Hi, really can't figure out this as it needs to be solved by hand (to be used orally). I only have an idea of how to nspire. Therefore, I hope there is a genius that can solve it in the head / or know the formula. Thanks in advance!

A normally distributed stochastic variable X has mean value µ and standard deviation σ. A sample of 100 measurements gave a 95% confidence interval, the width of which was 3.92

1) Determine σ
2) How many measurements should be made if the 95% confidence interval should have a width of no more than 2

2. ## Re: Confidence interval by hand

Always, with Normal Populations, $\displaystyle n = \dfrac{z^{2}\cdot s^{2}}{d^{2}}$

What is d?