One of the things we study in preparation of the PMP exam is "correlation". It's important for PMs to know how to quickly judge the relationship of two variables, say, for example, vendor performance on one deliverable versus overall project performance. As we know, pairs of variables can be highly correlated, unrelated, or negatively correlated, such as a higher budget overrun and your manager's level of happiness.
Well, as you see in the chart, there is a correlation between not only the years of PM employment and Salary, but if you look at the vertical distance between the two curves, there's also a consistent correlation of Salary depending on whether or not the population has a PMP(R) credential.
Say what you will about the exam, the process, even the PMI itself. In very practical terms - money - the PMP credential does seem to make a difference. Remember, this is a readout of population and not an individual statistic. So don't bring this into your boss' office and expect a raise if you just got your PMP. It's just as we said at the beginning - this shows that the PMP and Salary are related - not just friends or acquaintances.
Although this graph is