The most widely used profile shape functions are the pseudo-Voigt and the Pearson VII. Using the standard Mathematica (R) package "Statistics NonlinearFit" and high resolution synchrotron data, we have compared their convenience to numerical computation. With respect to this point, the pseudo-Voigt is better than the Pearson VII function. However the physical meaning of the pseudo-Voigt may be questioned since its variance is not defined. A good fit is obtained with a composite model, sum of a Pearson VII function with a finite variance and of a gaussian function. This gaussian function is assumed to described the thermal diffuse scattering.