def b(self, number):
        """A parameter to tune the amount of field length normalisation that is
        applied when calculating relevance scores.

        A value of 0 will completely disable any normalisation and a value of 1
        will fully normalise field lengths. The default is 0.75. Values of b
        will be clamped to the range 0 - 1.
        """
        if number < 0:
            self._b = 0
        elif number > 1:
            self._b = 1
        else:
            self._b = number