Statistics for Dummies by Deborah Jean Rumsey Page A

births versus the births of twins, triplets, and so on) for selected years from 1975–2000. Some questions that can be answered with this table are: What's the birth rate of males compared to females in Colorado? And is the rate of plural births changing? From this table, you can see that over a 25-year period, the percentage of female births remained steady at just under 49%, while the percentage of male births remained steady at just over 51%. (You may wonder why these percentages aren't closer to 50% each. This is a question for demographers — scientists who study human population trends — and biologists, not statisticians.) You can also see that the rate of plural births (as opposed to single births) seems to have changed over the years. It appears that the percentage of plural births is increasing, but which column do you look at: the number of plural births or the percentage of plural births? Does it matter? Yes, it does!
Table 4-2: Colorado Live Births by Sex and Plurality Status
Year
Total Number of Births
Number of Female Births
% Female Births
Number of Male Births
% Male Births
Number of Single Births
% Single Births
Number of Plural Births
% Plural Births
1975
40,148
19,447
48.4
20,701
51.6
39,385
98.1
763
1.9
1980
49,716
24,282
48.8
25,434
51.2
48,771
98.1
945
1.9
1985
55,115
26,925
48.9
28,190
51.1
53,949
97.9
1,166
2.1
1990
53,491
26,097
48.8
27,394
51.2
52,245
97.7
1,246
2.3
1995
54,310
26,431
48.7
27,879
51.3
52,669
97.0
1,641
3.0
2000
65,429
31,953
48.8
33,476
51.2
63,447
97.0
1,982
3.0
    Looking at percents versus totals
    How do you draw conclusions about trends in plural births over time by using the statistics presented in this table? If you look only at the number of plural births for 1975 compared to 2000, they increase from 763 to 1,982. Someone may try to say that this represents a 160% increase, or about 1.6 times as many plural births in 25 years ([1,982 – 763] ÷ 763). More plural births occurred in the year 2000 than in 1975, but more single births also occurred over this same time period. Because of this, the only accurate way to compare these statistics is to calculate the percentage of single versus plural births and compare these percentages. Looking at Table 4-2 , you can see that the percentage of plural births in 1975 was 1.9%, while in 2000 the percentage of plural births was 3.0%. You can conclude that the percentage of plural births did increase over time, even after taking the increased number of births into account. However, the increase is not 160%; it's closer to 58%: ([3.0 – 1.9]) ÷ 1.9) × 100%.
HEADS UP 
Beware of conclusions that are drawn from a data display that compares the number of individuals, as opposed to the percentage of individuals. Percentages represent a relative comparison of quantities (often over a period of time); this is usually an accurate way of comparing quantities, especially when the total number of items or events also changes over time. By looking at the percent change, you take into account the fact that the total number has also changed. (Of course, if you're truly interested in examining how the number of each item changes, you should look at the numbers and not the percentages.)
    Table 4-3 shows a breakdown of the number of live births in Colorado by the age of the mother for selected years from 1975–2000. The numerical variable age is broken down into categories that are of the same width (5 years) and are not overlapping. This makes for a fair and equitable comparison of age groups. However, the table gives only numbers of births in each case, so you can't look at the table and get a sense of any trends that may be developing over time in terms of the age of the mother. This problem can be solved by including the percents in parentheses along with the total number in each category, so that the reader can easily make a comparison. Another way to display the information is to include a pie chart for each year, showing the percent of the