The estimation of the variances of different estimators is always a crucial point in practical Monte Carlo calculations. The purpose of this Note is to formulate conditions that, in simplified situations, make track-length estimators more efficient than collision estimators for the estimation of reaction rates in a region. Starting from recent results of Amster and Djomehri in the first section of the Note, an upper limit is given for maximum extension of a nonmultiplying region. In the second section, assuming homogeneous medium and monoenergetic nonmultiplying transport with isotropic collision in the laboratory system, approximate conditions are described concerning the optical mean-chord-length of the region in terms of first-flight collision probabilities. Wigner rational approximation to the first-flight collision probability results in a surprisingly simple upper limit for the mean-chord-length of the region. Finally, the effect of the approximations to the results is discussed and lower and upper bounds, depending on the nonabsorption probability, are established for the reaction rate to be estimated. It is shown that, in practical cases, the approximations provide a lower value of the maximum extension still favorable from the viewpoint of the track-length estimator than the exact calculation.