To determine the accuracy of an estimate, it is common to calculate the percentage deviation (Dperc) from the real value (Joram et al., 2005). Using data from 615 students (5th and 6th grade) from a written estimation test for length, area, capacity and volume, disadvantages of Dperc were observed: The scale is closed to underestimations, which causes high skewness and high number of outliers (for overestimations). Internal consistency and discrimination power is rather low. Therefore, two alternatives were investigated: Dividing by the smaller value (estimated or real value, Dmin) proposed by Lörcher (2000), and logarithmic error score (Dlog), adapted from Clayton (1996)
