Non-linearity in the shear measurement. The standard linear-fit method is shown in the upper panel, using gridded simulations with 160 000 galaxies. The orange line indicates the fit. On the y-axis the difference between the average observed – or measured – shear and the true input shear is plotted. The lower panel shows the same data, but this time allowing for a quadratic term in the fit (fitting the function shown in Eq. (16)). Here the additional parameter α describes the coefficient of the quadratic term. The linear fit has 18 degrees of freedom, while the quadratic fit has 17.