Credit Score and Interest Rates: An inverse relationship?

Credit scores can make or break affordable payments on homes, cars, and pretty much anything we need to finance in life. But is there a standard scale for interest rates based on your credit score? 

I've been doing some research on bankrate.com (my go to resource) but haven't been able to find much research on the topic. I think the general consensus seems to be that lenders determine the types of risk premiums they will put on a loan based on their own set formula. 

However, I think MyFico.com does a pretty decent job on providing some general insights on how mortgage rates vary by different credit scores. For instance, I learned through the website that the APR, on a 30-year fixed-rate mortgage for a solid FICO score between 750 and 850 is roughly 3.2%, almost .4% below the current national average! That said, when you fall into the FICO range between 550 and 600, your APR on that 30-year fixed-rate mortgage can move up to nearly 6%.