The residuals are generally within one standard deviation of zero but do not appear randomly distributed. Instead there is a weak residual angular dependence. One possibility is that light beam, polarizers, and detector are not perfectly centered and the polarization is not completely uniform across the entire surface of the polarizer. Malus's law does not however appear to be in question. Rather, the residuals indicate a small level of polarizer-angle dependent systematic error.

chi-square test

Once the residuals have been calculated, the determination of the corresponding chi-square value for goodness of fit is straightforward. Here is an example: