code: def F_value(ER, EF, dfnum, dfden):
      
      return ((ER - EF) / float(dfnum) / (EF / float(dfden)))
docs: Returns an F-statistic given the following:
          ER  = error associated with the null hypothesis (the Restricted model)
          EF  = error associated with the alternate hypothesis (the Full model)
          dfR-dfF = degrees of freedom of the numerator
          dfF = degrees of freedom associated with the denominator/Full model
  
  Usage:   lF_value(ER,EF,dfnum,dfden)