Close but no cigar. Such is the life for Amgen's (Nasdaq: AMGN) Vectibix, which showed an improvement in survival of patients with head and neck cancer -- but not by a statistically significant margin.

Patients taking Vectibix plus chemotherapy survived for 11.1 months on average compared with nine months for patients taking just chemotherapy. The secondary measures of success also trended toward Vectibix, but it doesn't really matter as the overall survival wasn't statistically significant.

The two additional months of survival may be meaningful, but Amgen would have needed to run a larger trial to prove the difference wasn't just due to chance. For instance, if you flip a coin four times and you get 75% heads, you'd assume it was due to chance. If you flipped it 100 times and got 75% heads, you'd be more convinced that the coin was rigged.

The results are a little surprising because Vectibix's closest competitor -- Erbitux sold by Eli Lilly (NYSE: LLY), Bristol-Myers Squibb (NYSE: BMY), and Merck KGaA -- works in both colorectal cancer, which Vectibix is approved for, as well as head and neck cancer. When added to radiation treatment, Erbitux increased survival in head and neck cancer patients by almost 20 months, although the trial tested patients at an earlier stage of cancer development.

But Vectibix's disappointing results are only a little surprising for the simple reason that going after the same target is no guarantee of similar results. Bayer and Onyx Pharmaceuticals' (Nasdaq: ONXX) Nexavar and Pfizer's (NYSE: PFE) Sutent are both approved for kidney cancer. Nexavar works wonders in liver cancer, but Sutent was a flop when tested there.

The moral of the story here is that it's difficult to predict clinical trial successes and failures. Fortunately, Amgen has had some big wins lately with its bone drug denosumab, so investors shouldn't feel too bad about coming up a little short this time.

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Fool contributor Brian Orelli, Ph.D., doesn't own shares of any company mentioned in this article. Pfizer is a Motley Fool Inside Value selection. The Fool has a disclosure policy.