The investigation set out to determine whether there were significant changes in marking standard and reliability when eight experienced teachers carried out two successive assessments of the performance on a practical exercise using different random samples of 20 fourth‐year secondary school students on each occasion. The practical exercise required the students to prepare a salt by the reaction of an oxide with an acid, and the points of assessment concerned the production of the salt, and the performance of a filtration at an intermediate stage. The results showed a significant change in both the marking standard and the reliability for two of the four points assessed. However, the significant differences were observed only for the assessment of girls. More detailed analysis indicated that this sex bias occurred with both the men and women judge