This is the second article in a three-part series to help readers distinguish good metrics from bad. In part one we discussed good metrics. Here, we will look at a bad metric and consider how to change it into a useful, good metric. A bad metric is one that fails in one or more of the attributes of a good metric and is often not usable for the purpose it was intended.

Perhaps the reader recognizes d2 as slang for “designated driver,” but quality professionals will recognize it as a control chart constant used to estimate short-term variation of a process. The basic formula shown below is widely used in control charting for estimating the short-term variation using the average range of small samples. But what exactly is d2 and why should we care?

Choosing the correct linear regression model can be difficult. Trying to model it with only a sample doesn’t make it any easier. Let’s review some common statistical methods for selecting models, complications you may face, and look at some practical advice for choosing the best regression model.