If you do not have operational definitions for data provided in your organization, as you often will not, you need to figure out if the data is worth trusting (possibly by visiting the gemba) or whether you need to establish operational definitions and make sure new data is collected using the operational definitions.
Often the variation in the collection of data will hamper your ability to use PDSA. If you practiced PDSA as you should, this wouldn’t be a problem, because you would make sure data had operational definitions.
Without reliable data, the conclusions that will be drawn from results ads a great deal of potential for error to your effort.
continue reading: Data is Important and You Must Confirm What the Data Actually Says