Evidence-Based Practice for Nurses

Ungrouped Data Grouped Data

Age Tally Frequency Age Tally Percentage

18 ||| 3 18–19 |||| | 6

19 ||| 3 20–21 |||| | 6

20 | 1 22–23 |||| 5

21 |||| 5 24–25 | 1

22 ||| 3 > 25 || 2

23 || 2 Total 20

25 | 1

27 | 1

28 | 1

Total 20

TABLE 13-5 Mode of Age Data for 20 Subjects

change in just one data point affects the modality of the data. In the age data

presented in Table 13-3, note that the ungrouped data are unimodal with the

mode being 21. However, when the data are grouped, they become bimodal with

modes at 18–19 years as well as 20–21 years of age (Table 13-5). It is easy to see

the modes on the frequency polygon and histogram in Figure 13-1 because the

modes have the highest peaks and tallest bars. The mode is considered to be an

unstable measure of central tendency because it tends to vary widely from one

sample to the next. Given its instability, the mode is rarely presented as the sole

measure of central tendency.

Median

The median is the center of the data set. Just as the median of the road divides

the highway into two halves, the median of a data set divides data in half. If

there are an odd number of data points, the median is the middle value or the

score where exactly 50% of the data lie above the median and 50% of the data

lie below it. If there is an even number of values in the data set, the median

is the average of the two middle-most values and as such may be a number

not actually found in the data set. The median actually refers to the average

position in the data, and it is minimally affected by the existence of an outlier.

The position of the median is calculated by using the formula (n + 1)/2,

where n is the number of data values in the set. It is important to remember that

KEY TERMS

median: The point

at the center of a

data set

position of the

median: Calculated

by using the

formula (n + 1)/2,

where n is the

number of data

values in the set

338 CHAPTER 13 What Do the Quantitative Data Mean?