Clinical trial data comes in a variety of flavors, from "clearly doesn't work" to "clearly does."

And then there's Gilead Sciences' (NASDAQ:GILD) Zydelig, where the data is so clear that the Food and Drug Administration might as well rubber-stamp the approval without much more than a cursory look at the data.

A phase 3 trial testing Zydelig in patients with previously treated chronic lymphocytic leukemia was stopped early because patients receiving Zydelig plus Teva Pharmaceuticals' (NYSE:TEVA) Treanda and Rituxan, sold by Biogen (NASDAQ:BIIB) and Roche fared better than patients who received Treanda and Rituxan alone.

Data in a late-breaking abstract for a presentation at the American Society of Hematology meeting next week shows by just how much the tipple combination helps patients compared to the two drugs. The median progression-free survival -- how long patients survived without having their disease progress -- was 23.1 months for the triple combination, compared with 11.1 months for Treanda and Rituxan alone.

Do the statistics on that, and the p-value is 2.8 x 10^-14. Put another way, there's a 0.0000000000028% chance that the difference -- 23.1 months versus 11.1 months -- is due to chance alone. Part of the reason for the low p-value is that there were 207 patients in the Zydelig arm and 209 patients in the control arm. Gilead Sciences clearly could have gotten away with fewer patients and the same difference in progression-free survival and still showed statistical significance, but better to overshoot with too low of a p-value than have the clinical trial miss statistical significance.

While progression is a good indicator of whether a drug will work, overall survival is the gold standard. Unfortunately for those analyzing the data -- but fortunately for the patients -- the median overall survival hadn't been reached yet. But if you do the statistics on the data patients who have died, there's a 45% reduction in the risk of death by taking the triple drug combination, compared with the Rituxan and Treanda alone.

Expanding market
Zydelig is already approved to treat chronic lymphocytic leukemia, but it's limited to treatment in combination with Rituxan when other drugs aren't appropriate. The expanded market should help Gilead -- and Teva Pharmaceuticals, Biogen, and Roche -- sell more of their drugs. Gilead plans to ask regulators in the U.S. and EU to update the label with the new data early next year.

Teva Pharmaceuticals, Biogen, and Roche shouldn't get too excited about the potential bump in sales from Gilead Sciences' good data, though. Gilead is testing Zydelig as a first-line treatment before Treanda and Rituxan would be used, potentially making the triple combination obsolete fairly soon after it's approved.

Brian Orelli has no position in any stocks mentioned. The Motley Fool owns shares of and recommends Gilead Sciences. The Motley Fool recommends Biogen and Teva Pharmaceutical Industries. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.