It will be an interesting services, but it is hard to bet on stock based on stock picker's track record.

The Motley Fool, an investment and financial advisory services firm in Alexandria, Va., is expected to formally launch today a free online stock-research service dubbed CAPS (caps.fool.com) that will evaluate the effectiveness of thousands of stock pickers from the big Wall Street brokerages to individual investors. The service draws on the opinions of more than 7,600 investors who have provided ratings on more than 1,300 stocks. The service also tracks the performance of big brokerage firms, research boutiques and well-known stock pickers, such as James Cramer, former fund manager and a founder of TheStreet.com.

Here's how it works: Investors first predict whether a particular stock will beat or lag the S&P 500 stock-index over a set period. As those stocks change in value, participants are graded along two dimensions: how often they make a correct prediction and the percentage return on their stock picks compared with the S&P 500. The service also rates individual stocks, based on what the CAPS community thinks of them, using a five-star system with the highest-rated stocks receiving five stars. Investors with the highest rankings have a bigger influence on a stock's rating. The company began testing the service in the spring, first among Motley Fool employees, and later invited subscribers to its investment newsletters to contribute ratings.

The service is being launched at a time when firms are expanding their own stock-picking tools. In recent months, Charles Schwab Corp. began offering more stock research to more of its customers. The San Francisco brokerage plans to expand its stock-picking tools with redesigned stock screeners, more news coverage and detailed financials in the coming months.