Unless you're given to hanging out on the steps of the Securities and Exchange Commission and bribing postal workers, getting reliable data on companies traded over the counter has been as hopeless as tracing cash to the Contras. Too often, the limited information available through Standard & Poor's Corp. and Dow Jones & Co. tends to be selective, outdated, or just plain wrong. Major data vendors dabble in the better-known names among NASDAQ's exclusive National Market System (NMS), and even then your guess is better than theirs whether the historic price and capitalization they show, for example, reflects an arithmetic adjustment for a stock split.

Now, though, the mail will come to you, thanks to a couple of ambitious databases that gather the tidbits of financial reporting that stream ito the SEC in the form of 10-Qs (quarterly statements) and 8-Ks (material events). The two services, Market Screen, a product of Market Guide Inc., in Glen Head, N.Y., and Market Base, from MP Software Inc., in Needham Heights, Mass., package the updated figures company by company on microcomputer diskettes, and ship them posthaste to subscribers. Market Base sticks to 2,500 or so stocks on the NMS, considerably more than its competitors. Detailing more than 3,500 companies, Market Screen not only covers the lesser lights on NASDAQ, but hunts for signs of life on the obscure Pink Sheets, the appropriately tinted resume of bids and offers for companies so far too small or too bankrupt to qualify for listing anywhere else.

Prices start at a basic $295 and can run to $7,500 a year, depending on the coverage and frequency of delivery. For the fee, subscribers can filter up-to-date financials through appropriate criteria: all stocks with P/E ratios lower than 12, say, and debt to equity of less than 10%.

Unfortunately, to come up with winners you have to do some work on your own, and the two services vary considerably in convenience. While the more expensive Market Screen automatically supplies revenue and earnings growth over a three-year interval, for instance, Market Base makes you set up the calculation yourself. Market Screen's pro formas are more complete, and it links up with most spread-sheet formats. Indeed, Market Screen's data is so weighty that it takes up 4.5 megabytes of hard-disk memory.

But all told, despite thinner pickings that hardly strain an ordinary floppy, Market Base generally is better thought out (it deals in percentages as well as dollar amounts, for instance) and is more accessible to user ingenuity. Try asking for stocks with negative earnings in all but the most recent quarter, for example. A would-be investor can devise additional criteria of his or her own, thus constructing a unique performance score for each candidate.

Querying one database on January 2 to search for a company whose return on equity was greater than 20%, whose dividend yield was over 30%, and whose stock was selling for less than one times earnings produced one sure winner. Trouble was, it had just filed for reorganization and was folding up shop.