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Shares have since gained 52%, outpacing the Standard & Poor's 500 index's 40% improvement. But they've also swelled to a price/earnings-to-growth, or PEG, ratio — that's a stock's P/E ratio divided by its projected long-term earnings growth rate — of 1.06, from 0.77 at the time of our last story.

Is the prognosis for Express Scripts' stock still promising? We'll look into that today; the company turned up recently in our price/sales screen.

Earnings are the main driver of share prices over long time periods. So why run a value search based on the P/S ratio instead of the P/E ratio? Because sales are a more reliable measure of income than earnings. Earnings can fluctuate significantly from quarter to quarter due to one-time charges and credits. Sales, though, appear at the top of companies' income statements, before adjustments are made for such items.

Some argue that sales can also be a better predictor than earnings of share price movements. In his 1996 book "What Works on Wall Street," James O'Shaughnessy used 43 years' worth of market data to show that stocks with low P/S ratios had outperformed those with low P/E ratios, and that those with low P/S ratios and sales momentum had returned an average of 18.4% during his study period, vs. 13.0% for the broader market.

Our price/sales screen has produced some impressive performance figures of its own. Last June it drew our attention to Rayovac (ROV). We noted in our write-up of the company ("Batteries Sold Incessantly") that its P/S ratio of 0.75 was far lower than Energizer Holdings' (ENR) 1.34 and Gillette's (G) 4.61. Our look at the shares, which had already gained 111% in the past year, concluded that they had "higher still to charge." They're up 61% since our story, vs. 7% for the S&P 500 index.

Use our stock screener and the price/sales recipe anytime to run the search for yourself. Recently it produced a list of 10 candidates for further research, including Express Scripts.