Mutants and controls
All IMPC statistical analyses compare observations in genetic mutant animals against observations of wildtype controls.
The IMPC protocols guarantee that all mutant measurements are accompanied by corresponding data from matched non-mutant animals, i.e. from animals assessed at the same time and at the same facility. As a result, the IMPC has a large collection of measurements pertaining to wildtype animals, all of which are annotated with metadata.
Although the wildtype data have some variability, as power analyses have shown (https://www.nature.com/articles/ng.3360), statistical tests benefit from using a wide set of control data points. Thus, unless the control metadata indicate substantial changes, mutant lines are compared against controls from their immediate control group as well as many other relevant animals. In practice, the effective control pool can include up to several thousand wildtype animals.
Changes in metadata that disqualify pooling controls include differences in location (mutants are compared only against wildtype animals processed at the same facility).