Environmental Health Criteria 214

HUMAN EXPOSURE ASSESSMENT
Standard measures of dispersion include the sample variance, the
sample standard deviation and the sample range (formulas in Table 9).
These measures describe the spread of the observations. Examination of
Table 8 reveals that blood lead concentrations are more variable in
the Maltese sample population (standard deviation = 70.9 µg/litre,
range = 289 µg/litre) than that in Mexico (standard deviation = 49.5
µg/litre, range = 185 µg/litre). Measures of dispersion are useful for
characterizing the degree of variability of a given measure among the
members of a study population. As we will see later in this chapter,
dispersion is also a key component of some study design issues.
The concept of sample percentile is an important aspect of
exposure assessment. A sample percentile for a variable in a data set
is the value of the data such that at least p % are at or below this
value, and (1 - p)% are at or above this value. A percentile is
determined by first ordering the sample (i.e., rank from lowest to
highest) and then identifying the observation that corresponds with
the desired fraction of the data set. In the case of blood lead
concentrations measured in the Maltese sample population, 283 µg/litre
is the 75th percentile since it is the 27th of 36th rank-ordered
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