A study was designed to develop an instrument and methodological procedure to assess transitive relations within seriation problems in elementary school children using three criteria: explanations, judgments, and strategies. A secondary analysis to assess transitivity used the three criteria according to whether the children were conservers, in transition, or nonconservers of length. The instrument and methodology were developed from a Piagetian perspective. Each of the Kindergarteners, first and second graders participated in eight seriation tasks involving two sets of seriated rods, one set that varied in color and one set that was red. The one red rod from the array of colored rods was designated as the child's stick and placed within the second array of rods, behind a screen. Every child had to engage in the mental operations necessary to solve the problem of finding his red rod. Results indicate that children's explanations and strategies provide a richer data source than just judgments, for the comparison of performance at different grade levels. (CM)