Does Measurement Measure Up?: How Numbers Reveal and Conceal the Truth

CHAPTER 3: THE RATINGS GAME: ‘‘Overall’’ Measurements and Rankings

CHAPTER 3 THE RATINGS GAME ‘‘Overall’’ Measurements and Rankings Number Four should have been Number One . . . Thanks, honey. —Jack Dempsey (dedicating his autobiography to his fourth wife) In the world of measurement, there is a special kind of number, one that attempts to combine all of the various attributes of something into a single number. These types of numbers become popular because of their convenience . The Dow Jones Industrial Average is a good example. It is a single measurement representing the overall health of the American stock market. A relatively simple and useful example is the heat index, which indicates how the weather ‘‘feels’’ to a human being. If the temperature is 84\F, it feels worse outside if the humidity is 79% than if it were only 10%. (At 84\F and 79% relative humidity, the heat index is 94, while at 84\F and 10% relative humidity, the heat index is only 79.) The heat index is not a ‘‘perfect ’’ measure of how temperature and humidity combine to make us feel. For example, if it is 105\F and 10% relative humidity, the heat index is 100— the same as for 85\F and 85% humidity. These two conditions feel quite a bit different: compare Las Vegas or Phoenix summer weather, at 105/10, D OES M EASUREMENT M EASURE U P ? 38 with New Orleans or Houston at 85/85). So, the heat index is not perfect, but it is both useful and convenient. Emergency rooms know to expect more heat-related illnesses and disorders when the heat index exceeds, say, 100, and the general public can use the heat index forecast to plan outdoor activities, what to wear, and so on. Like some but by no means all of these overall measurements, the heat index has some foundation in scientific theory. The theoretical basis for the heat index relates to the mechanisms by which the human body cools itself when it is too hot. We perspire, and the moisture that forms on our skin then evaporates, resulting in the well-known and very powerful ‘‘evaporative cooling effect.’’ If you climb out of a swimming pool filled with 80\F water on a day when the air temperature is 95\F with 10% relative humidity, your skin will immediately feel cool—even though the air is much warmer than the water you just got out of. That’s evaporative cooling. The higher the humidity, however, the more difficult it is for our sweat to evaporate. Simply put, since the air is already full of water vapor, there is no place for our perspiration to go. (At 100% relative humidity, our sweat cannot evaporate at all.) The harder it is for our sweat to evaporate, the more difficult it is for us to cool down our bodies, and the hotter we ‘‘feel.’’ To return to the initial example, if it is 84\F, we feel much hotter if the relative humidity is 79% than if it is 10%. One often hears people say that they like the desert, because when they are there, they ‘‘don’t sweat.’’ This is untrue. If it is 100\F and 10% relative humidity, you will sweat quite freely, especially while exercising. However, your perspiration will evaporate rapidly in such hot, dry conditions, so fast that your clothes will not become wet, and it won’t seem to you as though you are sweating. That said, you really are sweating—and you can measure it. Weigh yourself before and after going for a run, playing tennis, and so on, in the desert. The weight you lose is almost entirely due to perspiration. Because measurements like the heat index that combine several factors into one number can be useful, they crop up all over the place. In some cases, these measurements are specifically designed to fulfill a special purpose . In others, they are simply measurements that over time have become popular as shorthand measures for some complex phenomenon. The Dow Jones Industrial Average (usually referred to simply as ‘‘the Dow’’) is a function of the average price of the stocks of 30 multibillion- T HE R ATINGS G AME 39 dollar American corporations. The Dow is named after Charles Dow, who created his first stock index in 1884. His purpose was to help investors (primarily) and stockbrokers (secondarily) make some sense out of the growing jumble of...