def xtqx(self):
        """get the normal matrix attribute. Create the attribute if
        it has not yet been created

        Returns
        -------
        xtqx : pyemu.Matrix

        """
        if self.__xtqx is None:
            self.log("xtqx")
            self.__xtqx = self.jco.T * (self.obscov ** -1) * self.jco
            self.log("xtqx")
        return self.__xtqx
get the normal matrix attribute. Create the attribute if
        it has not yet been created

        Returns
        -------
        xtqx : pyemu.Matrix