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calculate the F-statistic from two chi-square values

Calculate the F-statistic and its probability given new and old values of $\chi^2$and number of degrees of freedom (DOF).

Syntax: ftest chisq2 dof2 chisq1 dof1

The new $\chi^2$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). WARNING: this command can only be used if the extra model component is additive, this does not give the correct result if the component is multiplicative (see Orlandini et al. 2012, ApJ 748, 86).