164 Chapter 7 a larger number of patients were discharged than died. The standard deviation of S/F94 was also smaller in this sensitivity analysis (1.00, compared to 1.29 in the main analysis). The average change in S/F94 associated with a 15% reduction in risk of mortality was the same in main analysis and this sensitivity analysis (0.18), and correlation between S/F94 on day 0 and day 5 was almost identical in both analyses (0.31 in the main analysis compared to 0.32 in this sensitivity analysis). As a consequence of the lower mean and standard deviation of S/F94, the sample size required decreases from 1,444 in the main analysis, to 884 in this sensitivity analysis. Inclusion criteria We compared the effect different inclusion criteria had on required sample sizes (Supplementary Tables 2, 3 and 4). In general, as expected, the more broad the inclusion criteria (e.g. only age limits), the larger the required sample size; and the more specific the inclusion criteria, the smaller the require sample size. However, it is important to note that using more stringent inclusion criteria increases the burden associated with screening patients for inclusion in a study, which may substantially decrease real-world recruitment. n=550 n=550 n=550 n=78 n=550 A-a P/F ES baseline S/F 0.0 2.5 5.0 7.5 10.0 12.5 15.0 17.5 20.0 Absolute error (kPa) Minimum error n=4801 n=4801 n=4801 n=4800 n=4801 A-a P/F ES baseline S/F 0.0 2.5 5.0 7.5 10.0 12.5 15.0 17.5 20.0 Absolute error (kPa) Minimum error a) Including all recordings b) Restricted to recordings where SaO₂ < 0.94 Supplementary Figure 2. Predictive validity (ability to predict a future PaO2 after a change in FIO2; absolute error in kPa) of Alveolar- arterial (A-a), P/F ratio (P/F), effective shunt fraction (ES), and S/F ratio (S/F), compared to a baseline (the minimum achievable error, measured as the change in PaO2 between pairs of arterial blood gas measurements meeting the other selection criteria, with no change in FIO2). Lower values indicate superior validity. Centre line represents median values, box limits represent upper and lower quartiles, whiskers represent minimum and maximum values. All values are included in (a); in (b) only values in which SaO2<0.94 in the first measurement.
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