==== Front
Ann Saudi MedAnn Saudi MedAnnals of Saudi Medicine0256-49470975-4466King Faisal Specialist Hospital and Research Centre asm-2-156aLettersSeason change bias in animal studies Kolahi Jafar Mousavi Behrooz Adibi Shahriar Correspondence: Jafar Kolahi, Isfahan Science and Technology Town (ISTT), Odopharma Research Nucleus, No 10. Sayt 180. Shahin Shahr. Isfahan 83188-65161, Iran. Tel: +98-(312)-525 2209, kol_jaf@yahoo.comMar-Apr 2006 26 2 156 157 Copyright © 2006, Annals of Saudi Medicine2006This work is licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License.
==== Body
To the Editor: In research, bias includes any systematic error in the design, conduct, or analysis of the study. Bias can occur at all stages of research, from the selection of the population, how treatment is provided, to how and when outcome measurements are made.1 One report reviewed more than 50 possible source of bias in analytic research.2 Season change is a possible source of bias in animal studies that was not reported previously. We recently experienced this bias in evaluation of the biocompatibility of a new biomaterial with subcutaneous implantation in Sprague-Dawley white albino rat (male, mean weight, 200±25 g). During a longitudinal study, we obtained controversial results between summer and winter. All other variables were under control and we could not find any other source of bias. This source of bias could be attributed to hibernation in rodents,3 altering the immunological, pathological, and behavioral responses. By changing the environment temperature and daylight duration (combination of natural and artificial light, not artificial light only) it can be controlled, relatively. Nevertheless, in subtle studies both investigators and readers of the medical literature should consider this source of bias.
==== Refs
References
1 Jacob RF   Bias in dental research can lead to inappropriate treatment selection Dent Clin North Am 2002 46 1 61 78 11785746 
2 Sackett DL   Bias in analytic research J Chronic Dis 1979 32 51 63 447779 
3 Turning down the fires of life: metabolic regulation of hibernation and estivation Storey KB   Molecular mechanisms of metabolic arrest: life in limbo Oxford BIOS Scientific Publishers 2001 1 21