For each estimation approach (for details see text) and per deep venous thrombosis prevalence, the plot of the positive predictive value coverage probabilities and the 95% confidence interval width, for the calf difference test. 1 = Standard formula for obtaining a standard error of a proportion, 2 = as 1st approach but with correction for sampling fraction, 3 = Bootstrap procedure, 4 = Mercaldo and colleagues approach, 5 = Weighted logistic regression, 6 = Logit transformation of Mercaldo and colleagues approach). Colors/figures represent the different sampling fractions (Black circle = 1 case: 1 control, Red square = 1:2, Blue diamond = 1:3, Yellow triangle = 1:4). The vertical lines represent the ideal 95% coverage with its confidence interval, i.e. the levels of acceptability.