Experimental design.

<p>Each of six laboratories used 64 female mice of two inbred strains (C57BL6NCrl, DBA/2NCrl) ordered in two consecutive batches (n = 16 per batch and strain), with each batch being allocated to one experimental design. Upon arrival of a batch, the 16 mice per strain were randomly assigned to four cages in groups of four. To test heterogenization against standardization, we selected two experimental factors (test age, cage enrichment) and chose three factor levels A, B and C for each factor. Within each laboratory, the two factors were either standardized to factor level A (standardized design, uniform grey) or systematically varied across B and C using a 2×2 factorial design (heterogenized design, varying grey). According to the 2×2 factorial design of the heterogenized condition, study populations were divided into four blocks that were also characterized by similar microenvironmental differences due to cage position within the rack.