Operator error in the use of Powers roundness images has been studied. A logarithmic scale is proposed for the statistical analysis of roundness counts and determination of mean roundness and roundness standard deviation. Experimental error in determination of mean roundness from counts of fifty grains was + or -0.35 roundness class, a relatively large value considering the small range of mean roundness encountered. Pebble sphericities were determined by the three-diameter method; operator error here was virtually negligible, amounting to only + or -0.02 in a range of values from 0.41 to 0.85. Error in determination of phi median size and phi standard deviation by sieving were likewise exceedingly small, amounting to only + or -0.02phi in median diameter. This surprisingly small value for operator error is ascribed to the use of probability paper, as variation in determination of parameters was much greater when ordinary squared paper was used.