Comparison of baseline and follow-up urine mercury concentrations grouped by number of surfaces at baseline. Box and whisker plots showing distributions of urine mercury concentrations for the treatment group and the positive amalgam group (μg/g-creatinine). Boxes extend from the 25th to the 75th percentile, horizontal bars inside the boxes represent the median, whiskers extend to maximum and minimum observations within 1.5 times the length of the interquartile range above and below the 75th and 25th percentiles, respectively, and outliers are represented as circles. A wilcoxon signed-rank test was used for inter-group comparison and all were statistically significant except those indicated by a lower case letter (p < 0.05). Values >10 μg/g-creatinine have been removed for readability. * signifies an extreme outlier at 12 μg/g-creatinine.