Sampling by Attributes 



When the inspection is conducted to determine an attribute (propor- 

 tion of the population exhibiting a specific characteristic) , then the 

 sample size required to obtain a desired accuracy and level of confidence 

 is given by: 



n = z p(l - p) 



(9) 



where: z = standard normal distribution value associated with a 

 confidence coefficient of (l-a/2) 



c = desired accuracy of the estimate expressed as a percentage 



p = expected proportion of the population exhibiting the 

 attribute of interest 



a = desired confidence indicator 



Figure 1 shows the relationship between the factor p(l-p) and the 

 expected proportion (p) . 



Figure 1. Relationship of p(p - 1) versus p. 



