Document Detail

Design and analysis of pen studies in the animal sciences.
MedLine Citation:
PMID:  17517755     Owner:  NLM     Status:  MEDLINE    
Increasingly, research is being performed in which animals subjected to a common treatment are also housed in a common pen. Issues have been raised regarding the proper planning of experiments and conduct of statistical analyses in these instances. This paper reviews the problems associated with ignoring animal grouping during data analyses, and examples are provided for appropriate methods to use when animals are grouped in pens. Using animals as the error term when treatments are applied to pens can result in biased estimates of treatment effects when pens are of unequal sizes and animals are moved in and out of the pens. It always results in biased probability statements regarding their significance. The pen effect includes systematic effects other than that of the treatment, which is why pens must be replicated and randomized. In essence, pen studies have an implicit split-plot design in which the main plots (pens) receive the treatments of interest, whereas the subplots (cows) receive all the same subplot treatment. Using the subplot error to test main-plot treatment effects creates inflated degrees of freedom and uses the wrong denominator mean square to test the effect; hence, severely biasing the test of significance for the treatment effects and resulting in an invalid causal inference base. The interactions of pens with the fixed-effect elements of the treatment design are the correct error terms for those fixed-effects factors applied to the pens. The same statistical designs used with animals as experimental units can be used with pens. The number of experimental units to achieve a given power can be, and generally is, considerably less with pens because the variance among pens is generally less than the variance of cows within pens. Pens must be replicated, randomized, and included in the statistical model to ensure valid statistical inference.
N R St-Pierre
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Publication Detail:
Type:  Journal Article; Research Support, Non-U.S. Gov't; Review    
Journal Detail:
Title:  Journal of dairy science     Volume:  90 Suppl 1     ISSN:  1525-3198     ISO Abbreviation:  J. Dairy Sci.     Publication Date:  2007 Jun 
Date Detail:
Created Date:  2007-05-22     Completed Date:  2007-07-11     Revised Date:  2007-11-15    
Medline Journal Info:
Nlm Unique ID:  2985126R     Medline TA:  J Dairy Sci     Country:  United States    
Other Details:
Languages:  eng     Pagination:  E87-99     Citation Subset:  IM    
Department of Animal Sciences, The Ohio State University, Columbus 43210, USA.
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MeSH Terms
Animal Husbandry
Data Interpretation, Statistical*
Housing, Animal*
Randomized Controlled Trials as Topic
Research / methods*,  standards*,  statistics & numerical data
Research Design / standards*

From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine

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