Simulated power for sample size of 400 and allele frequency of 0.2. (Pattern is similar for other sample sizes and allele frequencies.) The relationship between FEV and confounders was modelled using the CAMP data and using the equations derived by Knudson et al.  Estimated power levels are for n trios simulated 10,000 times, with a type one error rate of 5 per cent. We simulated both family and population designs, each with and without truncation. Four methods of confounder adjustment were employed: FEV, PPFEV, residuals from regressing FEV on relevant covariates and residuals from regressing PPFEV on relevant covariates.