8. A manufacturer of cell phones batteries wants to estimate the useful life of its battery (in thousands of hours).The estimate needs to be within 0.10 (100 hours).Assuming a 95% level of confidence and a standard deviation of the useful life of the battery is 0.90 (900 hours). Determine the sample size. [Hint: First, identify the maximum allowable error from the above information and then apply the formula for estimating the sample size.]

How to Find a Sample Size Given a Confidence Interval and Width (unknown population standard deviation)

1. z_a/2: Divide the confidence interval by two, and look that area up in the z-table: .95 / 2 = 0.475. ...
2. E (margin of error): Divide the given width by 2. 6% / 2. ...
3. : use the given percentage. 41% = 0.41. ...
4. : subtract. from 1.