Threshold Pharmaceuticals' (Nasdaq: THLD) TH-302 is a good drug. It just isn't a super-duper extraordinary drug.

Apparently investors were looking for the latter and sent shares down 20% on Monday.

We already knew the drug delayed the progression of pancreatic cancer. Back in Febuary, Threshold said TH-302 combined with Eli Lilly's (NYSE: LLY) Gemzar produced a 63% improvement in progression-free survival compared with Gemzar alone.

This week, Threshold presented the full data package, including overall survival. Patients taking the higher dose of TH-302 combined with Gemzar lived 9.2 months compared to 6.9 months for Gemzar alone. While numerically longer, the results weren't statistically significant.

But they weren't designed to be. This was a phase 2 clinical trial with just 214 patients split into three different treatment groups. While overall survival was measured, it was only a secondary endpoint of the study, in part because patients that progressed on Gemzar alone were allowed to crossover and take TH-302. That may have increased the survival of the Gemzar-alone group.

With enough patients in the phase 3 trial, Threshold and partner Merck KGaA should be able to show statistical significance for the 2.3 month increase in survival.

The question, of course, is how much confidence we should have that the difference between the two treatment groups will remain that wide. The phase 2 data could have just happened by chance; that's the definition of not being statistically significant.

There are some hints that the difference might be real, though. The increase in progression-free survival is a good one. If TH-302 is delaying the growth of the tumor, there's a good chance that translates into an increase in survival, although it's no guarantee. For some types of tumors, Roche's Avastin showed increases in PFS but not overall survival. Ditto for Amgen's (Nasdaq: AMGN) Vectibix, Eli Lilly's Alimta, and Celgene's (Nasdaq: CELG) Thalomid as a maintenance therapy. Of course, there are more examples where the two correlated well than where they failed.

Another nugget of potentially good news is that a second lower dose tested in the phase 2 trial produced survival that was between the placebo and the higher dose. A dose response is a good sign that an effect is real.

Ultimately, it'll be the phase 3 trial that determines if TH-302 is good enough to get approved. It would've been nice if the drug produced survival data so wonderful that the phase 3 trial was an easy layup, but not all drugs can be of the super-duper variety.

If you want super-duper stocks, these three have an X factor that makes them stand out from their illustrious Dow peers. Find out what it is in the Fool's new free report, "The 3 Dow Stocks Dividend Investors Need."