by phase defect removal rate by phase defect removal leverage review rates process yield phase yield failure cost of quality (COQ) appraisal COQ appraisal/failure COQ ratio Planning and tracking Logging time, defect, and size data is an essential part of planning and tracking PSP projects, as historical data is used to improve estimating accuracy. The PSP uses the PROxy-Based Estimation (PROBE) method to improve a developer's estimating skills for more accurate project planning. For project tracking, the PSP uses the earned value method. The PSP also uses statistical techniques, such as correlation, linear regression, and standard deviation, to translate data into useful information for improving estimating, planning and quality. These statistical formulas are calculated by the PSP tool. Using the PSP The PSP is intended to help a developer improve their personal process; therefore PSP developers are expected to continue adapting the process to ensure it meets their personal needs. PSP and the TSP In practice, PSP skills are used in a TSP team environment. TSP teams consist of PSP-trained developers who volunteer for areas of project responsibility, so the project is managed by the team itself. Using personal data gathered using their PSP skills; the team makes the plans, the estimates, and controls the quality. Using PSP process methods can help TSP teams to meet their schedule commitments and produce high quality software. For example, according to research by Watts Humphrey, a third of all software projects fail, but an SEI study on 20 TSP projects in 13 different organizations found that TSP teams missed their target schedules by an average of only six percent. Successfully meeting schedule commitments can be attributed to using historical data to make more accurate estimates, so projects are based on realistic plans – and by using PSP quality methods, they produce low-defect software, which reduces time spent on removing defects in later phases, such as integration and acceptance testing. PSP and other methodologies The PSP is a personal process that can be adapted to suit the needs of the individual developer. It is not specific to any programming or design methodology; therefore it can be used with different methodologies, including Agile software development. Software engineering methods can be considered to vary from predictive through adaptive. The PSP is a predictive methodology, and Agile is considered adaptive, but despite their differences, the TSP/PSP and Agile share several concepts and approaches – particularly in regard