def covariance_factor(self):
    if (self.covfact in ['sc', 'scotts']):
        return self.scotts_factor()
    if (self.covfact in ['si', 'silverman']):
        return self.silverman_factor()
    elif self.covfact:
        return float(self.covfact)
    else:
        raise ValueError('covariance factor has to be scotts, silverman or a number')
def covariance_factor(self):
    if (self.covfact in ['sc', 'scotts']):
        return self.scotts_factor()
    if (self.covfact in ['si', 'silverman']):
        return self.silverman_factor()
    elif self.covfact:
        return float(self.covfact)
    else:
        raise ValueError('covariance factor has to be scotts, silverman or a number')