For the regression model in which an explanatory variable contains measurement error, the instrumental variable and the correction for attenuation estimators of slope are considered. Although each requires extra information that is rarely available in ideal form, examinations of asymptotic variances and simulation results suggest that the correction for attenuation performs as well as a fairly strong instrumental variable when the measurement errors are not too large, and this is often true even when the extra information acquired is rough. For larger measurement errors the correction method is of less value. Whenever both kinds of information are available, an estimator with smaller variance is possible by taking a weighted average of the two.