Design Strategies for Longitudinal Spirometry Studies Study Duration and Measurement Frequency

Measuring the longitudinal change in FEV1 is useful for assessing the adverse effects of respiratory exposures and pulmonary diseases. Investigators seek to estimate the “true” mean FEV1 slope ( μβ) of an infinite population. The difference between the estimated mean FEV1 slope (^μ β) and the true mean slope, resulting from biological variation and measurement errors, can be minimized by increasing the number of subjects (N), years of follow-up (D), or the frequency of measurements (P). We defined maximum error emax such that P[ ‖μβ − μβ‖ ⩽ emax] = 0.95, and thus emax is one-half the width of the 95% confidence interval for μβ. We computed the values of emax on the basis of actual data obtained from 160 coal miners and working nonminers who had completed 11 spirometry measurements, using recommended equipment and procedures, at 6-mo intervals over 5 yr. Individual 5-yr FEV1 slopes ( Δ FEV1) were calculated by linear regression. For a range of values of N, D, and P, tables are provided for emax, the magni...