Coordinator
May 3, 2011 at 5:50 PM
Edited May 3, 2011 at 5:52 PM

The left and right probabilities are Pvalues. Suppose the distribution of a test statistic (t.Distribution) looks like this:
XX
XXXY
XXXXXYYY
^
(That's supposed to look like a tdistribution.) If the actual value of the test statistic (t.Statistic) is where the caret is, then the one sided tail probability of that test statistic value is the area of Ys, which is t.RightProbability.
So the code to compute a onesided tail probability is:
double oneSidedP;
if (t.Statistic < 0.0) {
oneSidedP = t.LeftProbability;
} else {
oneSidedP = t.RightProbability;
}
If you are asking for the twosided tail probability, that is:
double twoSidedP;
if (t.Statistic< 0.0) {
twoSidedP = 2.0 * t.LeftProbability;
} else {
twoSidedP = 2.0 * t.RightProbability;
}
Do let me know if these values aren't agreeing with your table values. They certainly should be.
For tests that are often used twosided, we should probably add CentralProbability and TailProbability methods to the TestResult.
