It's a good thing that Amarin's (NASDAQ:AMRN) investors are used to waiting because the Food and Drug Administration delayed another decision.

Last month, the agency told Amarin that it was delaying a decision on whether to expand the use of its lipid-lowering drug, Vascepa, and would make a decision January 15 on the biotech's appeal of the FDA's decision to rescind Amarin's Special Protocol Assessment.

The SPA is supposed to make it easier for biotechs to get their drugs approved by getting the FDA's requirements for approval in writing before the clinical trial begins. Amarin lived up to its end of the bargain -- proving that Vascepa lowers triglycerides and substantially enrolling an outcomes study -- but the FDA said trials run on AbbVie's (NYSE:ABBV) Tricor, Merck's (NYSE:MRK) Cordaptive, and others showed it was possible for a drug to lower triglyceride levels without improving cardiovascular outcomes. The FDA decided that the SPA was no longer valid, which made it unlikely the FDA would approve Vascepa for patients with moderately high triglyceride levels.

On Wednesday, the FDA told Amarin that it wasn't ready to make a decision. When might it be ready? The FDA has wised up and didn't give Amarin a date.

This is good news
Because it was the FDA that made the decision, and the FDA that will make a decision on the appeal, I figured the agency would respond with a quick, "Yep, we were right." The fact that the agency is taking longer means there's actually a chance the FDA could change its mind.

The skeptic could argue that the decision to reject the appeal could already be made, but there's an advantage to taking longer to announce the decision because it makes the agency look less authoritarian.

But shares are down?
Compared to yesterday's close, they are, but I think that has more to do to with short-term traders exiting since the date of the binary event is unknown. Amarin has been on a nice run since the FDA said it was still considering the appeal.

AMRN Chart

AMRN data by YCharts

Of course, if you zoom out a little more to before it became clear the FDA would cancel the SPA, the chart gets really ugly.

AMRN Chart

AMRN data by YCharts

Handicapping the decision
While the decision didn't happen yesterday, the FDA will eventually make a decision about the SPA and the expanded approval for Vascepa. We can use Amarin's 52-week highs and lows as likely prices after the binary event to see how likely investors think it is that the FDA will change its mind.

52-week high * likelihood of approval + 52-week low * likelihood of rejection = current price

Note: The likelihood of rejection is equal to (1-likelihood of approval). If there's a 60% chance of approval, there's a 40% chance of rejection, making it possible to solve for the likelihood of approval. Isn't binary event math grand?

Solving for the likelihood of approval we get:

Likelihood of approval = (current price-52-week low) / (52-week high-52-week low)

Or likelihood of approval = (current price-1.36) / 6.79

At today's closing price of 2.26, investors are giving Amarin about an 11% chance of getting an approval. If you think there's a greater chance of approval, Amarin is a good buy at that price according to my calculations.

I should note that the 52-week high is a little conservative, because even when it hit that level, there wasn't a 100% guarantee of an approval; but let's call it a margin for error. There's no guarantees the 52-week low will hold if the FDA rejects the appeal.

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Fool contributor Brian Orelli has no position in any stocks mentioned. The Motley Fool has no position in any of the stocks mentioned. 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.