Chapter 26. Chapter 26 Graphic Content

According to this figure, in the United States the percent of adults infected with HIV is 0.5% to 0.9%. In order to estimate the number of adults infected with HIV, we need to estimate the number of adults in the United States. Assuming that there are about 300 million people in the United States (there were actually 313 million), we might estimate that three-quarters of people in the United States, or 225 million people, are adults. (The proportion of people in the United States who are under 18 years old is actually about 24% of the 313 million people, or 238 million.) If there are 225 million adults in the United States, and 0.5% to 0.9% are infected with HIV, that works out to 1.125 million for the low estimate and 2.025 million for the high estimate. If we had to give one number, it would probably be best to take the average of these two numbers: 1.575 million.

It is difficult from this map and the associated data to determine which countries have the largest number of adults infected with HIV. Certain countries, those shown in dark red, clearly have a much higher percentage of adults infected with HIV than other countries. But the data shown here are meant to convey information about which countries have the highest proportion of their population infected with HIV. In order to know which country has the most infected adults, it is necessary to multiply the rate of infection in each country by the number of adults. But those data are not given. And so, at a glance, we cannot easily identify which countries have the largest number of adults infected with HIV.

Using the same logic as in the previous question/answer, from the information given it is not possible to determine whether there are twice as many adults infected with HIV in the United States as in Canada. The rate of infection in Canada is between 0.1% and 0.49% (or, to take the midpoint, approximately 0.3%). And in the United States the rate of infection is between 0.5% and 0.9% (or, to take the midpoint, approximately 0.75%). This suggests that the rate of infection in the United States is about twice as high as in Canada. Given that the number of adults in the United States is much greater than in Canada, however, that would lead us to conclude that the total number of adults infected with HIV in the United States is more than double the number of infected adults in Canada.

From the map and associated data, it appears that the rate of adult HIV infection in South Africa is between 15% and 28%. (To take a midpoint, we could say 21.5%.) In South America, most of the countries have infection rates of 0.5% to 0.9%, although there appears to be at least one country with a rate of 1.0% to 4.9% and at least one with a rate of 0.1% to 0.49%. (To take a midpoint, we could say 0.75%.) From these data, we could conclude that the rate of HIV infection among adults is significantly higher in South Africa than in South America. Further, we could conclude that the rate of infection is almost 30 times higher in South Africa.

The different colors used to indicate successively increasing rates of infection of adults with HIV in the map do not represent consistent increases. Rather, each of the last three intervals is considerably broader than the previous, and each of those intervals is much larger than the first three intervals. While this might confuse or mislead a reader who does not carefully note the infection rates designated by each color, it serves an important purpose. In many cases, it is important to be able to convey a very large range of numbers. When doing so, it can be helpful to indicate when one data point is double the previous data point. And when the next data point is double that one. In this figure, the ranges used with each color make that possible. This makes it possible to show fine gradations at the low end, while also showing meaningful gradations at the high end, even when the high end is 100 times greater than the low end.