Free Statistics for Dummies by Deborah Jean Rumsey Page B

Without a placebo, the researchers could not be certain that the results were due to the actual effect of the treatment, because some (or all) of the observed effect could have been due to the placebo effect.
    Blind and double-blind
    A blind experiment is one in which the subjects who are participating in the study are not aware of whether they're in the treatment group or the control group. In the zinc example, a placebo would be used that would look like the zinc pill, and patients would not be told which type of pill they were taking. A blind experiment attempts to eliminate any bias in what the study subjects might report.
    A double-blind experiment controls for potential bias on the part of both the patients and the researchers. Neither the patients nor the researchers collecting the data know which subjects received the treatment and which ones didn't. A double-blind study is best, because even though researchers may claim to be unbiased, they often have a special interest in the results — otherwise they wouldn't be doing the study!
    Surveys (polls)
    A survey (more commonly known as a poll ) is a measurement tool that is most often used to gather people's opinions along with some relevant demographic information. Because so many policymakers, marketers, and others want to "get at the pulse of the American public" and find out what the average American is thinking and feeling, many people now feel that they cannot escape the barrage of requests to take part in surveys and polls. In fact, you've probably received many requests to participate in surveys, and you may even have become numb to them, simply throwing away surveys received in the mail, or saying "no" when you're asked to participate in a telephone survey.
    If done properly, a survey can really be informative. People use surveys to find out what TV programs Americans (and others) like how consumers feel about Internet shopping and whether the United States should have a nuclear defense system. Surveys are used by companies to assess the level of satisfaction their customers feel, to find out what products their customers want, and to determine who is buying their products. TV stations use surveys to get instant reactions to news stories and events, and movie producers use them to determine how to end their movies.
    If I had to choose one word to describe the general state of surveys in the media today, I'd have to use the word quantity , rather than quality. In other words, you'll find no shortage of bad surveys. You can ask a few basic questions to determine whether a survey has been conducted properly; these issues are covered in detail in Chapter 16 .
    Estimation
    One of the biggest uses of statistics is to guesstimate something (the statistical term is estimation ), as in the following examples:
What's the average household income in America?
What percentage of households tuned in to the Academy Awards this year?
What's the average life expectancy of a baby born today?
How effective is this new drug?
How clean is the air today, compared to ten years ago?
    All of these questions require some sort of numerical estimate to answer the question, yet the business of coming up with a fair and accurate estimate can be quite involved. The following sections cover major elements in that process. For more information on making and interpreting estimates, see Chapter 11 .
    Margin of error
    You've probably heard someone report, "This survey had a margin of error of plus or minus 3 percentage points." What does this mean? All surveys are based on information collected from a sample of individuals, not the entire population. A certain amount of error is bound to occur — not in the sense of calculation error (although there may be some of that, too) but in the sense of sampling error , or error that's bound to happen simply because the researchers aren't asking everyone. The margin of error is supposed to measure the maximum amount by which the sample results are expected