In order to compare the longitudinally and cross-sectionally determined annual decline of spirometric measurements, we measured spirograms from 326 male adults four times over five years. Acceptable results were obtained three times or more in 269 subjects aged 30 to 55 at the initial survey. Longitudinal annual changes in the height-squared proportional values of forced vital capacity (CFVC), forced expiratory volume in one second (CFEV1), and maximal expiratory flow at 50% and 25% of FVC (CVmax 50 and CVmax 25), was estimated by the model in which the effect of each individual's level was included as an explanatory variable. The cross-sectional annual change in those indices was determined by including the effect of each survey's level in the model.
The longitudinal estimate of annual decline was significantly smaller than the cross-sectional estimate for all indices except CVmax 25. No evidence suggested that systemic error of measurement or a learning effect caused singificant bias in the data. The discrepancy in the estimated annual changes seemed to be caused by the cohort effect in our subjects, since the cross-sectional analysis is primarily sensitive to harmful factors operating in the past. We concluded that the longitudinal data for individuals or groups should not be compared with any reference value based on a cross-sectional analysis.