Recent advances in statistical methods allows analysis units to be other than the experimental units to facilitating analysis of dependent data and exploring witin unit variation. In the absence of controlled characteristics of sujects or experimental condition, comparison of treatments must account for potential interactions. Exposure, grow-out took place in unit of tank. However, tank membership changes from exposure to transferring, and to grow-out. Had this hadn't been the case, the experimental unit would have been tanks.
Mated mother exposed to the test compound at several dose levels include a control.
At term fetuses are examined for various developmental anomalies. Often assessed through manipulated covariates, or observation. Examples include age, gender, physical condition, genetic strain if identifiable, socio-economic condition, climate, temperature, etc. Variation from these sources is not part of the treatment effects, but may influence the outcome synergistically or antagonistically with the treatment. Unfortunately all uncontrolled variation is also dumped there.