Returns the (1-tailed) significance level (p-value) of an F
statistic given the degrees of freedom for the numerator (dfR-dfF) and
the degrees of freedom for the denominator (dfF).

Usage:   lfprob(dfnum, dfden, F)   where usually dfnum=dfbn, dfden=dfwn

def fprob(dfnum, dfden, F):
    """
Returns the (1-tailed) significance level (p-value) of an F
statistic given the degrees of freedom for the numerator (dfR-dfF) and
the degrees of freedom for the denominator (dfF).

Usage:   lfprob(dfnum, dfden, F)   where usually dfnum=dfbn, dfden=dfwn
"""
    p = betai(0.5 * dfden, 0.5 * dfnum, dfden / float(dfden + dfnum * F))
    return p