Search Engine Optimization is a subject with many unknown variables. Google doesn’t divulge how their search engine works and many attempt to solve the mystery. Because of that alone there are a lot of hypotheses that are often unproved. The way to test those hypotheses is to perform a sequence of experiments like Commission Blueprint did with their PR to content correlation test. They have tested loads of sites over a period of a year. This kind of test is statistically significant and proves the speculation. In this example, by the way, the link of PR and content was proven to be non-existent.