# Thread: poisson question with ti 89

1. ## poisson question with ti 89

lambda = .55, p(y <= 1)

poissoncdf(.55, 1, infinity)

Why do I keep getting a domain error with that?

2. ## Re: poisson question with ti 89

Hey cokedude.

I think you should use the fact that P(X > a) = 1 - P(X <= a).

Also - I don't know what the arguments for the function are but I can tell you that you can use this trick to find things like this out with distributions like the one you have [i.e. Poisson].

3. ## Re: poisson question with ti 89

The poissoncdf( command takes two arguments:
The mean is the average number of times the event will happen during the time interval we're interested in.
The value is the number of times we're interested in the event happening (so the output is the probability that the event happens at most value times in the interval).
See The poissoncdf( Command - TI-Basic Developer

4. ## Re: poisson question with ti 89

Originally Posted by chiro
Hey cokedude.

I think you should use the fact that P(X > a) = 1 - P(X <= a).

Also - I don't know what the arguments for the function are but I can tell you that you can use this trick to find things like this out with distributions like the one you have [i.e. Poisson].
In my case I want p(y <= 1), not P(X > a). Would the X = 1, or would the a = 1?

5. ## Re: poisson question with ti 89

The a would be 1.