Statistics for Dummies Read Online Free PDF

leftovers. The story reported that 28% of the people surveyed said they microwave leftovers almost daily, 43% said they microwave leftovers two to four times a week, and 15% said they do it once a week. Assuming that everyone should fit into these results somewhere, the percentages should add up to 100, or close to it. Quickly checking, the total is 28% + 43% + 15% = 86%. What happened to the other 14%? Who was left out? Where do they fall in the mix? No one knows. The statistics just aren't adding up.
    Four out of five — really?
    Another item you can check quickly is whether the total number of respondents is given. As a quick example, you may remember the Trident gum commercials which said that "Four out of five dentists surveyed recommend Trident gum for their patients who chew gum." This commercial is quite a few years old, but recently it has been revived in a funny series of new commercials asking, "What happened to that fifth dentist?" and then showing some incidents that might have happened to the fifth dentist that stopped him or her from pushing the "yes" button. But here is the real question: How many dentists were really surveyed? You don't know, because the survey doesn't tell you. You can't even check the fine print, because in the case of this type of advertising, none is required.
    Why would knowing the total number of respondents make a difference? Because the reliability of a statistic is, in part, due to the amount of information that went into the statistic (as long as it was good and correct information). When the advertisers say "four out of five dentists", there may have actually been only five dentists surveyed. Now, maybe 5,000 were surveyed, and in thatcase, 4,000 of them recommended the gum. The point is, you don't know how many dentists actually recommended the gum unless you do more detective work to find out. In most cases, the burden is on you, the consumer, to find that information. Unless you know the total number of people who took part in the study, you can't get any perspective on how reliable this information could be.
    Uncovering misleading statistics
    Even when you uncover an error in a statistic, you may not be able to determine whether the error was an honest mistake, or if someone with an agenda was conveniently stretching the truth. By far the most common abuse of statistics is a subtle, yet effective, exaggeration of the truth. Even when the math checks out, the underlying statistics themselves can be misleading; they could be unfair, stretch the truth, or exaggerate the facts. Misleading statistics are harder to pinpoint than simple math errors, but they can have a huge impact on society, and, unfortunately, they occur all the time.
    Crime statistics that don't pay
    When spotting misleading statistics, you want to question the type of statistic used. Is it fair? Is it appropriate? Does it even make practical sense? If you're worried only about whether the numbers add up or that the calculations were correct, you could be missing a bigger error in that the statistic itself is measuring the wrong characteristic.
    Crime statistics are a great example of how statistics are used to show two sides of a story, only one of which is really correct. Crime is often discussed in political debates, with one candidate (usually the incumbent) arguing that crime has gone down during his or her tenure, and the challenger often arguing that crime has gone up (giving the challenger something to criticize the incumbent for). How can two political candidates talk about crime going in two different directions? Assuming that the math is correct, how can this happen? Well, depending on the way that you measure crime, it would be possible to get either result. Table 2-1 shows the number of crimes in the United States reported by the FBI from 1987 to 1997.

Table 2-1: Number of Crimes in the U.S. (1987–1997)
Year
Number of