Account valuation
A previoius sample had standard deviation $8,200.
How many accounts should be examined to estimate the mean account
valuation to within $1,000 with probability 0.95?
The diagram finds the sample size by a trial-and-error method. For any values of n and s, it finds the width of the resulting confidence interval.
Use the bottom slider to set the standard deviation to $8,200. Then drag the sample size slider and verify that n should be 260 or higher to get the '±' value below 1000.
Solving an equation
You could also mention that, provided n is reasonably large (so that the t-value will be approximately 1.96), the equations can be solved to give
A discount brokerage selected a random sample of 36 customer accounts. Their mean total account valuation was $32,000 with standard deviation $8,200.
The resulting 95% CI ($32,000 ± 2,780) was not accurate enough.