Product quality is usually measured by the number of "bugs" that a software product may have. Two metrics typically used in this context are defect density, defect rate and Defect Injection Rate
Defect Density: (at System Testing Stage) Total number of defects identified during system testing / Actual size of the component
Defect Rate : It is the expected number of defects over a certain time period specified. It is important for cost and resource estimates of the maintenance phase of the software life cycle.
Defect Injection Rate: # of defect introduced (aggregated by root cause)/ Total defects
