Choosing the precision when estimating a reliability
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The precision is based on the width of the confidence interval for the parameter you are estimating. The wider your confidence interval, the fewer units you need to test. By default, Minitab calculates a one-sided confidence interval, so the width only affects the distance between the estimate and the lower or upper bound. You can request a two-sided confidence interval in the Options subdialog box.

For example, if you want to estimate the reliability of your units at 200 hours, and the lower bound is to be a reliability that is no more than 0.025 below your estimate, choose Lower bound and enter 0.025 as your desired precision in Sample sizes or precisions as distances from bound of CI to estimate.

You may want to enter a range of values for the precision, to see its impact on your sample size.