The ratio of the number of defects and the production size in which they were detected is known as the 'pd' level of defects. This index is dimensionless or expressed as a percentage (pd %). It refers to gross production and ranges from 0-1. The index is related only to a given portion of production and not to the time period over which production takes place. However this fact is important but is usually neglected when analysing defect levels which may be recorded on a daily, weekly, monthly or annual basis. This paper strives to show that specific periods of production in which the defects are detected should be considered. In calculating the ratio of defects to production size, the time factor is not taken into account but the index changes show that the length of time over which measurements are taken is essential.