Evidence-Based Practice for Nurses

range, percentile, and standard deviation. z scores and variance are used to
compare the variability among data using different units of measure.

Range

A statistical range is the difference between the maximum and minimum values
in a data set. Because the range is very sample specific, it is considered to be
an unstable measure of variability. Table 13-7 shows the income reported for
subjects in the experimental and control groups of a study designed to evaluate
health literacy. The mean for both data sets is $48,000, and the medians are
each $42,000. However, the data are quite different because the data for the
experimental group are more heterogeneous than for the control group. The
experimental group’s income varies from a minimum of $15,000 to a maxi-
mum of $103,000, with a range of $88,000. The control group’s income has a
minimum of $30,000 and a maximum of $73,000. Although salaries are still
quite variable, they are more uniform than the experimental group’s salaries
because their range is $43,000. In data comparisons, the smaller the range,
the more uniform the data; the greater the range, the more variable the data.

Semiquartile Range

Just as the median divides data into two halves, other
values divide these halves in half, meaning into quarters.
The semiquartile range is the range of the middle 50%
of the data. Figure 13-6 illustrates the semiquartile range
of the age data from a previous example. The semiquartile
range lies between the lower (first) and upper (third)
quartile of ages. The first quartile value is the median
of the lower half of the data set; one fourth of the data

KEY TERMS

measures of

variability:

Measures providing

information about

differences among

data within a

set; measures of

dispersion

homogenous:

Elements that share

many common

characteristics

heterogeneous:

The degree to

which elements are

diverse or not alike

range: The

difference between

the maximum and

minimum values in

a data set

semiquartile range:

The range of the

middle 50% of the

data

Control Experimental

$30,000 $15,000

$38,000 $23,000

$42,000 $42,000

$57,000 $57,000

$73,000 $103,000

Range $43,000 $88,000

TABLE 13-7 Example of Ranges of Income

FYI

The most commonly reported measure of

variability, the standard deviation, is the

measure of the average deviations of a value

from the mean in a given data set. Because

standard deviations are based on the mean

of the data set, information collected using

different measurement scales cannot be

directly compared.

13.5 Measures of Variability 345