Читать книгу Design for Excellence in Electronics Manufacturing - Cheryl Tulkoff - Страница 40

Statistics are used to describe samples of populations. If the sample size is small, every member of the population can be tested, and statistics are not needed. Consider ahead of time whether the tool or technique being applied to a data set is appropriate. Many statistical tools are valid only if the data is random or independent. In most cases, samples must be chosen randomly – every member must have an equal chance of being selected. When testing to failure, if the testing is ended before all items have failed, the sample is not random. In this case, the data is censored. Type I censoring is time‐based. Type II censoring is failure‐based. Censored data can be analyzed using special techniques. Testing of a small lot of parts is typically not effective for detecting issues occurring at a rate below 5% of the population, and the ability to detect all quality and reliability issues by testing alone is simply not practical. This is especially true when the primary objective is the lowest possible cost.