Sample Size
The question always arises: “How many samples should be used in the study?” To the dismay of most people, the answer is “enough”. The purpose of any measurement study (variables or attribute) is to understand the properties of the measurement system.. A sufficient number of samples should be selected to cover the expected operating range (see also Chapter II Section C). With attribute measurement systems, the area of interest are the Type II areas (see Chapter I, Section B). If the inherent process capability is good (i.e., large Cp, Cpk or Pp, Ppk) then a small random sample may not have many (or any) samples in this area. This means that as the process capability improves , the required random sample for the attribute study should become larger).