Statistics for Dummies by Deborah Jean Rumsey Page B

total births that were born to women in each of the eight nonoverlapping age groups.

Table 4-3: Colorado Live Births by Mother's Age
 
 
Age of Mother (Years)
----
Year
Total Number of Births [ * ]
10–14
15–19
20–24
25–29
30–34
35–39
40–44
45–49
----
1975
40,148
88
6,627
14,533
12,565
4,885
1,211
222
16
----
1980
49,716
57
6,530
16,642
16,081
8,349
1,842
198
12
----
1985
55,115
90
5,634
16,242
18,065
11,231
3,464
370
13
----
1990
53,491
91
5,975
13,118
16,352
12,444
4,772
717
15
----
1995
54,310
134
6,462
12,935
14,286
13,186
6,184
1,071
38
----
2000
65,429
117
7,546
15,865
17,408
15,275
7,546
1,545
93
----
[ * ] The sums of the births may not add up to the total number of births, due to unknown or unusually high age (50 or over) of the mother.

    Note that because the totals are reported in this table, you can do the work of finding the percentages on your own, if you wanted to. (Had the table presented only percents, without any totals, you would have had an easier time comparing percents. But you would have been limited in the conclusions you could have drawn, because you would not have known the total numbers.) Just to save you time, I calculated those percents for you, for the combined group of mothers aged 40–49. Table 4-4 shows those calculations. From this table, you can see that a trend in mother's age appears to be emerging. More women are having babies in their 40s than before, and the percentage is steadily increasing.

Table 4-4: Percent of Colorado Live Births to Mothers Aged 40-49
Year
Total Births
Number of Births to Mothers Aged 40–49 Years
% of Births to Mothers Aged 40–49 Years
----
1975
40,148
238
0.59%
----
1980
49,716
210
0.42%
----
1985
55,115
383
0.69%
----
1990
53,491
732
1.4%
----
1995
54,310
1,109
2.0%
----
2000
65,429
1,638
2.5%

    The footnote to Table 4-3 (paraphrased from the note originally written by the Colorado Department of Public Health and Environment) indicates that any mothers who were aged 50 or older were not included in this data set. Recent studies suggest that a growing (albeit still small) percentage of women are having babies in their early 50s, so this data set may have to be augmented in time to include that age group.
    Putting percents into perspective
    Don't be fooled into thinking that just because certain percentages are small, they aren't meaningful and/or comparable. All of the percentages in Table 4-4 are small (equal to or less than 2.5%) but the percentage for the year 2000 (2.5%) is over four times the percentage for 1975 (0.59%), and that represents a very large increase, relatively speaking. Similarly, don't assume that when a large percent increase is reported, the situation involves a large number of people. Suppose someone announces that the rate of a particular disease quadrupled over the past few years. That doesn't mean a large percentage of people are affected, it means only that the percentage is four times as large as it used to be. The percentage of people affected by the disease in question may have been extremely small to begin with. An increase is still an increase, but in some situations, reporting the percentage alone can be misleading; the prevalence of the disease needs to be put into perspective in terms of the total number of people affected.
REMEMBER 
A percentage is a relative measure. However, look for the total number, as well, to keep the actual amounts in proper perspective.
    Keeping an eye on the units
    Sometimes, tables can be a bit confusing if you're not watching carefully. For example, the IRS reports "Tax Stats at a Glance" on its Web site, and some of those statistics (reported exactly as the IRS did) are shown in Table 4-5 .

Table 4-5: Statistics on Individual Income Tax Returns
Number of Returns (FY2001)
129,783,221
----
Gross Collections (FY2001 in millions of dollars)
1,178,210
----
Top 1% AGI break (TY1999)
$293,415
----
Top 10% AGI break (TY1999)
$87,682
----
Bottom 10% AGI break