The F Distribution is commonly used to analyze variances, particularly ANOVA. The data are never negative, so the graph is a distorted bell curve. The middle of the curve is not at zero.
CalGraph will calculate the chance under an F Distribution, that is, the probability that the variable lies between two values.
Click on Statistics at the top of the window and then on F Distribution. This window opens.
Type the lower number in the F1 box, the higher number in the F2 box, the first Degrees of Freedom in the D.F.1 and the second Degrees of Freedom in the D.F. 2 box. Then click Calculate.
Here CalGraph shows that the chance the F variable with 20 Degrees of Freedom in the numerator and 30 Degrees of Freedom in the denominator is between 0.75 and 100 is 0.746165. The graph shows the area under the curve shaded.
Probably you want the area above 0.75; that is from 0.75 to infinity. To 6 decimal places there is no difference between the area from 0.75 and 100 and that from 0.75 to infinity. When the Degrees of Freedom are very small (like 1, 2 or 3), to get the chance to infinity put 99999999999 in the F2 box.
You might want the area below some value, say below 60. Because Chi Square is always positive, this means from 0 to 60. Put 0 in the X1 box and 60 in the X2. Hit Calculate.
You can print the graph and your result by clicking File and then Print. Or you can copy it to another document, using Edit > Copy Graph to Clipboard and then Paste in that document.
CalGraph will also calculate the other way,
Given an area, CalGraph will find the Critical Value which cuts off that area. At the top of the F Distribution window, click on Critical Values and then Alpha.
Type in the two Degrees of Freedom. Type the area you know in the alpha box and click Calculate.
Here when the numerator has 9 Degrees of Freedom, the denominator has 12 Degrees of Freedom and 0.05 is the area of the region under the curve between 2.80 and infinity.
CalGraph will calculate the p-value for a hypothesis test.
CalGraph will not calculate the F Distribution if either of the Degrees of Freedom is above 80.