This tool lets you calculate the probability that a random
variable X is in a specified range, for a variety of probability
distributions for X: the normal
distribution, the binomial
distribution with parameters *n* and *p*, the
chi-square
distribution,
the exponential distribution,
the geometric
distribution, the
hypergeometric distribution,
the negative
binomial distribution,
the
Poisson distribution, and
Student's *t*-distribution.

The first choice box lets you select a
probability distribution. Depending on the distribution you
select, text areas will appear for you to enter the values of the
parameters of the distribution. Parameters that are probabilities
(*e.g.*, the chance of success in each trial for a
binomial distribution) can be entered either as decimal numbers
between 0 and 1, or as percentages. If you enter a probability as
a percentage, be sure to include the percent sign (%) after the
number.

The check boxes and corresponding text entry areas let you constrain the range of values for which to find the probability. If a box is not checked, there is no constraint in that direction. For example if the box in front of >= is not checked, the lower limit of the range of values for which the probability is calculated is minus infinity; if the box in front of <= is not checked, the upper limit of the range of values for which the probability is calculated is infinity. When a box is checked, the value in the corresponding text area limits the range of possibilities for which the probability is calculated.

The tool also shows the expected value and standard error of the distribution.