A simple method measuring new effective lines of code showed that between 19 and 40 percent of code written on three projects wasn't in the final release. Generally, productivity is a function of input effort and output size. A strong understanding of software productivity, coupled with a good estimate of software size, is key to predicting project effort and, ultimately, producing reliable project duration estimates, schedules, and resource needs. Project managers and engineers often measure or predict the size of released software-the volume of software in the marketed product. However, the final release doesn't include reworked code-code that was changed or deleted during development.