Start with Data Collection Plan

To establish procedure to calculate your process performance’s metrics, you will need a proper set of tools. I would recommend this : start from creating a data collection plan (DCP).

A comprehensive DCP will consist of the following : Voices of Customer (this in turn, will be adopted as the key measurement, the metrics), Critical Customer Requirement, Operational Definition (of each CCR), Formula to calculate the current base line (of each CCR), the type of the data to be collected (i.e. discrete vs continuous), last but not least : how much data to be collected, from where, when, and by who.