In the present paper, a critical approach to the evaluation of Schmidt hammer testing is attempted. Through a statistical analysis of field measurements on sandstone blocks from the archaeological site of Delphi (Greece), it was shown that there is an inconsistency in the correction method proposed by the manufacturer, in relation to the direction of rebound. The horizontally obtained rebound values, when corrected, yielded lower values than the vertically obtained ones. When examining the various factors potentially responsible for possible differences between the vertical and horizontal values, we conclude that the proposed correction method is not applicable to all cases and that there are many factors that may influence the end results.