This is a proprietary Morningstar data point.

The Morningstar Rating for Stocks is calculated by comparing a stock's current market price with Morningstar's estimate of the stock's fair value. Our rating system also includes an uncertainty adjustment, so that it's more difficult for a company to earn a 5-star rating the more uncertain we are of our fair value estimate.

Under our system, 3-star stocks are those that should offer a "fair return," one that adequately compensates for the riskiness of the stock. Three-star stocks should offer investors a return that's roughly comparable to the stock's cost of equity. (The cost of equity is often called a "required return" because it represents the return an investor requires for taking on the risk of owning the stock.)

Five-star stocks, of course, should offer an investor a return that's well above the company's cost of equity. Conversely, low-rated stocks have significantly lower expected returns.

The Morningstar Rating for Stocks also includes a small buffer around the cutoff between each rating, to reduce the number of rating changes produced by random market "noise." If a $50 stock moves up and down by $0.25 each day over a few days, the buffer will prevent the star rating from changing each day based on this insignificant change.