Calculate the F-statistic and its probability given new and
old values of and
number of degrees of freedom (DOF).
Syntax: ftest chisq2 dof2 chisq1 dof1
The new and DOF, chisq2 and dof2, should come
from adding an extra model component to (or thawing a frozen parameter of) the
model which gave chisq1 and dof1.If the F-test probability is low then it is
reasonable to add the extra model component. WARNING : it is not correct to use
the F-test statistic to test for the presence of a line (see Protassov et al
2002, ApJ 571, 545).