Two new error-in-variables methods to estimate the reactivity ratios are developed. These are based on variants of the Skeist solution of the copolymerization equation. One method is suitable when one of the reactivity ratios is close to one and the other is for use when both ratios are close to one. In simulations, the latter is compared with an error-in-variables method based on the original Skeist solution. The results of the two error-in-variables methods and the extended Kelen-Tudos method are given for a wide selection of reactivity ratios and error magnitudes.