#### KitchenAlbatros

##### Registered

I'm trying to perform a Ppk study. (= my employer wants a Ppk number)

But as I was busy with collecting the data and calculating the standard deviation. I came across this question my coworker asked me.

Why don't you use a pooled standard deviation?

Here is how the data that I'm working with is organised.

On one production line we have every month 3 times production of product X, 5 times product Y, 1 time product Z. (for example)

My take on the ppk study was to aggregate all data, calculate stdev (overall) and then the ppk.

My coworker says that when using a pooled stdev the stdev from the product that is produces the most has more of an impact on the overall ppk. And I follow him on this.

But my question remains, what is the best approach? aggregating all data or using a pooled stdev?

Thanks for any input.