Reliability Physics and Engineering pp 419-441 | Cite as

# Sampling Plans and Confidence Intervals

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## Abstract

Before purchasing a large number of devices, a customer will likely ask the supplier about the *defect level* for the product being offered. The customer’s reliability inquiry is often expressed as: what is the defect level for the *population* of such devices in terms of number of defective devices per hundred, number of defective devices per thousand, number of defective devices per million (dpm), etc.? To determine the *fraction defective*, a sample of the devices is randomly selected from the population and this sample is tested/stressed to determine the fraction defective. After the fraction defective is determined for the sample, then it is only natural to ask: based on the *sample size* used, what is the *confidence interval* for the population fraction defective? To answer this critically important question, we must understand the basics of *sampling theory.*

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