Cohort Analysis

In The Lean Startup the concept of cohort analysis is that (engagement) metrics are defined per group of customers that comes into contact with your product independently. The technique is useful to measure if similar groups are exhibiting different behavior and therefore 'prove' that your product investments result in different (buying) behavior.

As an example: if product developments do not lead to increase measurable product usage, workflow improvements, throughput increases maybe the investments where unneccessary.