Fig. 2.

Linear mixed models analysis for comparison of FVC and FEV1 values obtained with seven different spirometers. Mean differences with Bonferroni corrected 95% confidence intervals and p values from linear mixed models analysis are shown for comparison of FVC (a) and FEV1 (b) and FEV6 (c) for seven different spirometers; 4 hot-wire (HW1–HW4), two ultrasonic (US1–US2) and one wedge-bellows (WB). Linear mixed models analysis was performed with a random intercept for subjects and a fixed effect for the type of spirometer used. A model with 3 modalities (HW, US, WB) was used for group wise comparisons of the different types of spirometers, and a model with 7 modalities (HW1, HW2, HW3, HW4, US1, US2, WB) was used for comparisons of all spirometers against each other. This set up was used for FVC, FEV1 and FEV6, resulting in six different linear mixed models. Bonferroni corrections were performed for 95% confidence intervals and p values for each model. Significance level is set at 0.05