Russel-Research-Method-in-Anthropology

Univariate Analysis 579
countries to the right. In fact, the mean is $5,447 and the median is just
$1,302.
And we can now see that the variable TFR is distributed bimodally. This is
especially evident in the frequency polygon, which has the telltale two-hump
shape of a bimodal distribution. Many countries appear to have low TFR or
high TFR, with fewer countries in the middle range.
Bimodal Distributions
Obviously, if we calculate the median or mean for a bimodal variable, we
won’t get a realistic picture of the central tendency in the data. To understand
TFR better, I divided the data into cases above the median of 3.14 and cases
below it. Then I calculated the means and medians for each of the two sets of
data and ran the frequency polygons.
Figure 19.7a shows the frequency polygon for the cases above the median;
figure 19.7b shows the polygon for the cases below the median. Now it looks
Number of Cases
8
7
6
5
4
3
2
1
0
3 4 5 6 7
a. TFR
Number of Cases
8
7
6
5
4
3
2
1
0
1.0 1.5 2.0 2.5 3.0
b. TFR
Figure 19.7. Frequency polygons for TFR (in table 19.8) above the median (figure
19.7a) and below the median (figure 19.7b).
as though we may have a multimodal distribution. This might be the result of
sampling (which means we always have to be cautious about interpreting pictures
of variables), but it isn’t. TFR really is multimodal. I was able to find
out by running the same analysis on 187 countries of the world for which we
have data on this variable and comparing the shapes. Usually, this is out of the
question.
Bimodal and multimodal distributions are everywhere. Figure 19.8 shows
the frequency polygon for LEXFEM, female life expectancy, in table 19.8.