Scrutinizing the Theory of Comparative Time Studies with Operator as a Block Effect

Abstract The existence of considerable productivity differences between operators is well known in forestry work studies. Several techniques have been developed to manage operator (i.e. inter-individual) effects and thus enable general conclusions to be drawn. In the Nordic countries inter-individual variations have generally been managed by using “within-operator” comparisons. The methodology is equivalent to the statistical method of blocking, when defining each operator as a block effect. Unfortunately this approach has traditionally been referred to as “comparative studies,” although it only addresses one of many possible components of genuine comparative work studies. In the traditional motivation for usage of operator blocking it is assumed that productivity relationships between work methods are independent of the operator if the same operator works with both methods. Hence, comparisons of relative productivity should ignore inter-individual variations, enabling universal productivity relationships between methods/conditions of interest to be determined. Unfortunately this assumption has often been taken literally, with expectations of productivity relationships to be identical between individuals instead of on a population level. In this article the literal approach is scrutinized using the time taken by 12 operators to undertake various tasks in an experimental study of firewood processing with different machine systems and round wood types. Operators, as a population, did respond in a similar manner to treatments, but there was great variation between individuals. Hence, the assumptions of literally uniform individual productivity relationships should, therefore, be replaced by a population based theoretical foundation, which justifies the continued use of operator blocking to objectively handle the inevitable operator effect in forestry work studies.