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Dana B. Barr,¹ Doug Landsittel,² Marcia Nishioka,³ Kent Thomas,⁴ Brian Curwin,⁵ James Raymer,⁶ Kirby C. Donnelly,⁷ Linda McCauley,⁸ and P. Barry Ryan⁹

¹National Center for Environmental Health, Centers for Disease Control and Prevention, Atlanta, Georgia, USA; ²Department of Mathematics and Computer Science, Duquesne University, Pittsburgh, Pennsylvania, USA; ³Battelle Memorial Institute, Columbus, Ohio, USA; ⁴National Exposure Research Laboratory, Office of Research and Development, U.S. Environmental Protection Agency, Research Triangle Park, North Carolina, USA; ⁵National Institute for Occupational Safety and Health, Centers for Disease Control and Prevention, Cincinnati, Ohio, USA; ⁶RTI International, Research Triangle Park, North Carolina, USA; ⁷Texas A&M University System Health Science Center, College Station, Texas, USA; ⁸School of Human Environmental Sciences, University of Pennsylvania, Philadelphia, Pennsylvania, USA; ⁹Rollins School of Public Health, Emory University, Atlanta, Georgia, USA

Abstract
Developing internally valid, and perhaps generalizable, farmworker exposure studies is a complex process that involves many statistical and laboratory considerations. Statistics are an integral component of each study beginning with the design stage and continuing to the final data analysis and interpretation. Similarly, data quality plays a significant role in the overall value of the study. Data quality can be derived from several experimental parameters including statistical design of the study and quality of environmental and biological analytical measurements. We discuss statistical and analytic issues that should be addressed in every farmworker study. These issues include study design and sample size determination, analytical methods and quality control and assurance, treatment of missing data or data below the method's limits of detection, and post-hoc analyses of data from multiple studies. Key words: analytical methodology, biomarkers, laboratory, limit of detection, omics, quality control, sample size, statistics. Environ Health Perspect 114: 961–968 (2006) . doi:10.1289/ehp.8528 available via http://dx.doi.org/ [Online 16 February 2006]

This article is part of the mini-monograph "Farmworker Exposure to Pesticides: Methodological Issues for the Collection of Comparable Data."

Address correspondence to D.B. Barr, CDC, 4770 Buford Hwy., Mailstop F17, Atlanta, GA 30341 USA. Telephone: (770) 488-7886. Fax: (770) 488-0142. E-mail: dbarr@cdc.gov

We thank the organizers of the Farmworker Pesticide Exposure Comparable Data Conference held in Winston-Salem, NC, on 30 September–1 October 2004 and for which this document was generated. The conference organizers were T. Arcury, S. Quandt, L. McCauley, J. Hoppin, D. Barr, and M. Robson. We also thank other participants in the workshop who provided input into this document.

We thank those who provided financial support for the conference: Pesticide Environmental Trust Fund ; N.C. Department of Agriculture and Consumer Services ; the National Institute of Environmental Health Sciences and the National Institute for Occupational Safety and Health (R13 ES/OH013378) ; and CropLife America Inc.

The findings and conclusions in this report are those of the authors and do not necessarily represent the views of the Centers for Disease Control and Prevention. Although this work was reviewed by the U.S. Environmental Protection Agency and approved for publication, it may not necessarily reflect official agency policy.

The authors declare they have no competing financial interests.

Received 11 July 2005 ; accepted 16 February 2006.