Environmental Health Criteria 214

HUMAN EXPOSURE ASSESSMENT
lead measurements from the Maltese and Mexican sample populations. The
bottom and top horizontal lines of each box denote the interquartile
range (i.e., the 25th and 75th percentiles) and the solid horizontal
line across the centre indicates the sample median. The dotted line
across the box indicates the mean of the distribution. The whiskers on
the boxes in Fig. 12 extend to the 10th and 90th percentiles of the
distributions, and the open circles denote all observations beyond
those percentiles. As illustrated here, box plots are a convenient
method for displaying information on the central tendency, dispersion,
symmetry and tails of an exposure measure.
4.2.2.4 Quantile-quantile plots
Quantile-quantile plots can be used to compare the distribution
of a variable with a different sample or a known distribution.
Exposure measures are commonly compared to the normal or lognormal
distribution (see section 4.3) for purposes of evaluating whether the
normality assumptions inherent in numerous statistical inference
methods are met. While a discussion of probability distributions and
statistical inference methods is reserved for later in the chapter, a
quantile-quantile plot is shown in Fig. 14. Here, the Maltese blood
lead data are plotted against the standard normal distribution (see
section 4.3). This special form of quantile-quantile plot is known as
a normal probability chart. Data that form an approximately straight
line on such a chart are approximately normally distributed. Data that
do not form a straight line follow a non-normal probability
distribution.
4.2.2.5 Scatter plots
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