Editing Personal software process (section)

==The importance of data==
One of the core aspects of the PSP is using historical data to analyze and improve process performance. PSP data collection is supported by four main elements:
* Scripts
* Measures
* Standards
* Forms

The PSP scripts provide expert-level guidance to following the process steps and they provide a framework for applying the PSP measures. The PSP has four core measures:
* Size – the size measure for a product part, such as lines of code (LOC).
* Effort – the time required to complete a task, usually recorded in minutes.
* Quality – the number of defects in the product.
* Schedule – a measure of project progression, tracked against planned and actual completion dates.

Applying standards to the process can ensure the data is precise and consistent.
Data is logged in forms, normally using a PSP software tool. The SEI has developed a PSP tool and there are also open source options available, such as Process Dashboard.

The key data collected in the PSP tool are time, defect, and size data – the time spent in each phase; when and where defects were injected, found, and fixed; and the size of the product parts.  Software developers use many other measures that are derived from these three basic measures to understand and improve their performance. Derived measures include:

* estimation accuracy (size/time)
* prediction intervals (size/time)
* time in phase distribution
* defect injection distribution
* defect removal distribution
* productivity
* reuse percentage
* cost performance index
* planned value
* earned value
* predicted earned value
* defect density
* defect density 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