Introductory Biostatistics

intervals are

10–19

20–29

30–39

the midpoint for the first interval is

9 : 5 þ 19 : 5

2

¼ 14 : 5

and for the second interval is

19 : 5 þ 29 : 5

2

¼ 24 : 5

This process for calculation of the meanxusing Table 2.3 is illustrated in Table
2.7.

xF

2086 : 5

57

¼ 36 :6lb

(If individual weights were used, we would havex¼ 36 :7 lb.)
Of course, the meanxobtained from this technique with a frequency table is
di¤erent from thexusing individual or raw data. However, the process saves
some computational labor, and the di¤erence between the results,x’s, is very
small if the data set is large and the interval width is small.
As indicated earlier, a characteristic of some interest is the symmetry or lack
of symmetry of a distribution, and it is recommended that for very positively

TABLE 2.7

Weight
Interval

Frequency,

f

Interval

Midpoint,mfm

10–19 5 14.5 72.5
20–29 19 24.5 465.5
30–39 10 34.5 345.0
40–49 13 44.5 578.5
50–59 4 54.5 218.0
60–69 4 64.5 258.0
70–79 2 74.5 149.0

Total 57 2086.5

74 DESCRIPTIVE METHODS FOR CONTINUOUS DATA