Investment Studio > Expressions > Functions > Statistical > TDIST

float tdist(float x, integer degrees_of_freedom, integer tails, boolean cumulative = TRUE)

Returns Student's t probability function.

x is the point of evaluation.

degrees_of_freedom >= 1 is the number of degrees of freedom.

tails Î [1, 2] is the number of distribution tails to include in the result when cumulative = TRUE.

If cumulative = TRUE, the complement of the CDF (Cumulative Distribution Function) is returned (equal to 1 - CDF, i.e. the probability that x is < a stochastic variable with Student's t distribution); otherwise, the PDF (Probability Density Function) is returned. If omitted, cumulative defaults to TRUE.

The Student's t distribution is the distribution of the ratio of a standard normal (see normsdist) and a c² (see chidist) stochastic variable. It's used in hypothesis testing when samples are small or the population variance is unknown (see linest, logest, ttest). As the number of samples (and hence degrees_of_freedom) grows, the t distribution converges with the standard normal distribution.

Example

=tdist(1.96, 100, 2)

returns 0.052778901366089.

See also normsdist, tinv, ttest, ztest.