The actual selection of the guess function to be used for the least squares fit depends on the nature of the experimental data. So far, we have studied only least squares polynomials. However, many cases arise when data from experimental tests is exponentially related or tends to have vertical and horizontal asymptotes, so we need to consider guess functions other than polynomials. In this section, we broaden our discussion of least squares approximations to some popular forms of guess functions that arise when data are plotted.