Nonselection and nonresponse bias in clinical research.

In summary, nonselection and nonresponse bias can have a potent impact on the validity of clinical veterinary research studies and should be carefully assessed by investigators and readers. The risk of nonselection and nonresponse bias has been compared to "lowering yourself into a dark pit and trusting you won't be bitten by a snake ... before you go into the pit, you should stand outside and listen for a hissing sound ... if you hear one, do not go on ... if you do not hear anything, you may proceed with caution, being confident that at worst you will be bitten by a quiet snake."