Environmental Health Criteria 214

HUMAN EXPOSURE ASSESSMENT
Scatter plots display the relationship between two exposure
variables measured from the same unit of observation (e.g., a person
or location). Scatter plots are useful for graphically illustrating
associations that are summarized numerically by correlation
coefficients. Possible results include noticeable linear or non-linear
trends, the absence of trend (a big "cloud") or a general trend with
some observations as outliers. Outliers are observations that do not
follow the trends of the rest of the data and can strongly affect
estimates of associations by masking real effects. Outliers can be the
result of measurement error, human error or a correct but abnormal
observation. Regardless, all potential outliers should be checked for
accuracy and corrected or removed if this is justifiable. Fig. 15
contains a scatter plot of blood lead and faeces lead measurements
made concurrently on the Maltese sample population. Note that the plot
indicates a positive association between the two measurements, but
that the relationship is not 1 : 1, i.e., a unit change in blood lead
levels is not accompanied by a constant change in blood faeces
concentrations. This observation is consistent with the correlation
coefficient between these measures of 0.57 that was noted in the
previous section.
4.3 Probability distributions
Most exposure measurements can be considered random variables;
that is, the different values obtained for a measurement of a given
type are a function of a set of causative variables that may or may
not be known to the analyst (Ott, 1995). Statistics allows for
analysis of random variables by incorporation of variation through
probability. This addition of variation allows for the generalization
of results to populations larger or different than the sample under
consideration.
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