Implications of experimental technique for analysis and interpretation of data from animal experiments: outliers and increased variability resulting from failure of intraperitoneal injection procedures.

Intraperitoneal (i.p.) injections are widely used in laboratory animal experiments. This technique has a failure rate that is typically reported to be of the order of 10-20%. It is not apparent that failures of i.p. injection and their consequences for the experimental results are as widely recognized as the use of the technique. We illustrate the consequences of i.p. injection failure for the analysis and interpretation of several bioassays. We suggest approaches to data analysis that should be considered, and emphasize the need to recognize and allow for the possibility of i.p. injection failure in the analysis and interpretation of laboratory animal experiments involving this technique.