Poisson Probabilities; Cumulative Poisson Probabilities - Texas Instruments TI-83 Plus Manual

Find the probability that at most 2 or less of these 10 packages will not arrive at its destination
within the specified time.
nd
Press the 2 key and then press the VARS key
to get to the DISTR page.
Press the ALPHA key and then press the MATH key
to get to the letter A which is the binomcdf( function.
After binomcdf( appears on the screen, type in 10, .07, 2).
Press the ENTER key.
The result is 0.9716578543 or ≈ 97.2%.

Poisson Probabilities

The procedure to find probabilities associated with a Poisson distribution on your calculator are
very similar to the ones needed for the binomial probabilities. The function poissonpdf( is also
found on the DISTR page directly under the binomcdf( function.
To find the probability that a random variable from a Poisson distribution with mean λ, type
poissonpdf(λ, x).
Example: Telemarketing
Suppose that a household receives, on the average, 9.5 telemarketing calls per week. We want to
find the probability that the household receives 6 calls this week.
Type poissonpdf(9.5, 6); the probability is 7.64%.

Cumulative Poisson Probabilities

As with the binomial distribution, there is a command poissoncdf( for computing cumulative
Poisson probabilities. The poissoncdf( function is also located on the DISTR page. To find the
probability that a Poisson random variable with mean λ will take a value at most x, type
poissoncdf(λ, x). To find the probability that the random variable will lie between a and b, type
This will add the probabilities for values up to b, and then subtract off the probabilities for values
below a.
poissoncdf(λ, b) − poissoncdf(λ, a − 1)
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