An Input Metric measures what you did to get a desired result.
An Output Metric measures your output.
I wanted a way to measure 'how extreme' my extreme programming is, so I can measure the output and correlate the answers to see if I'm doing any good.
Gathering the metrics themselves must a lightweight agile procedure, or people will feel bogged down.
ShodanInputMetric is a technique I offer for lightly measuring your XP process.
After having done that with my small team over the past year, we've come up with Xpef,
or Extreme Programming Evaluation Framework.
Read more about this stuff at XpEvaluationFramework.