title: A method is presented to plan the required sample size when estimating regression-based reference limits.
abstract: OBJECTIVE
  Reference limits are widely used in anthropometry, the behavioral sciences, medicine, and clinical chemistry. They describe the distribution of a quantitative variable in a healthy population, and are often a smooth function of age or another determinant. Thus, instead of estimating reference limits separately for several age groups, it is more economical and parsimonious to use regression methods to estimate reference limits as a function of age. Although the variability of regression-based reference limits has been addressed previously, the available methods to determine the sample sizes needed to estimate them are neither transparent nor user-friendly.
  
  
  METHODS
  We propose a simple and intuitive formula using margins of error, to project the sample sizes required to achieve a given degree of precision, for different sampling strategies.
  
  
  RESULTS
  We present two examples for the calculation of the sample size required to estimate a specific reference limit using various age distributions.
  
  
  CONCLUSIONS
  We provide a simple formula to calculate the sample size needed to estimate a specific reference limit to a specified degree of precision. The structure of the formula can easily accommodate different age-sampling strategies.
authors.affiliation: Department of Epidemiology and Biostatistics, McGill University, Montreal, Quebec, H2T 1H3, Canada. bellera@bergonie.org
authors.forename: Carine A
authors.initials: CA
authors.lastname: Bellera
authors.forename: James A
authors.initials: JA
authors.lastname: Hanley
mesh_terms: D017677:Age Distribution; D001794:Blood Pressure; D002648:Child; D016001:Confidence Intervals; D015340:Epidemiologic Research Design; D006801:Humans; D008433:Mathematics; D012044:Regression Analysis; D018401:Sample Size