Brian,

It's not quite that simple. I wouldn't have come up with the stat issue again, but since you asked the question, let me try to answer it:

of course you are right with your calculations, this is the very low probability that anyone out of the GENERAL population will get "twitch-onset" ALS.

However, for a person who suddenly develop twitching we can no longer

take the general population as a basis. Here comes conditional probability

into play. For this person, the real question is: "How are the odds of ALS,

given the fact that I already have one of the symptoms (i.e widespread

twitching) Now we must take as "basis population" the subgroup of twitchers instead of the whole population.

(That's why it would be so important to know the actual frequency of BFS

in the population)

Fortunately, though, it is reasonable to assume that BFS is indeed quite common, (my own estimate would be about 1 Percent)

So we could take the 2,890,000 twitchers (out of 289,000,000 general population) as the basis which leads to a higher but still very reassuring low odds for ALS.

Furthermore, if you factor in other facts like clean EMG and neuro exam,

then, of course, the odds are still much lower

Let me add that this post is really not meant to sound priggish/know-it-all-ish (I hope it isn't perceived in that way

I just wanted to clarify the so-often misunderstood issue about (conditional) probability...