Introductory Biostatistics

If five intervals are used, we would have

w¼

67

5

¼ 13 : 4

and if 15 intervals are used, we would have

w¼

67

15

¼ 4 : 5

Between these two values, 4.5 and 13.6, there are two convenient (or

conventional) numbers: 5 and 10. Since the sample size of 57 is not large,

a width of 10 should be an apparent choice because it results in fewer

intervals (the usual concept of ‘‘large’’ is ‘‘100 or more’’).

2. Since the smallest number is 12, we may begin our first interval at 10. The
   considerations discussed so far lead to the following seven intervals:

10–19

20–29

30–39

40–49

50–59

60–69

70–79

3. Determining the frequencies or the number of values or measurements
   for each interval is merely a matter of examining the values one by one
   and of placing a tally mark beside the appropriate interval. When we do
   this we have the frequency distribution of the weights of the 57 children
   (Table 2.2). The temporary column of tallies should be deleted from the
   final table.


4. An optional but recommended step in the formulation of a frequency
   distribution is to present the proportion orrelative frequencyin addition
   to frequency for each interval. These proportions, defined by

relative frequency¼

frequency

total number of observations

are shown in Table 2.2 and would be very useful if we need to compare

two data sets of di¤erent sizes.

60 DESCRIPTIVE METHODS FOR CONTINUOUS DATA