Featur e


Watch Your Weights


The Key to Accurate Surveys By Matthew DeBell, Ph.D.


tant insights, while a bad one is money down the drain. Even worse, when a bad survey is mistaken for a good one, the cost of errors resulting from the flawed data can easily dwarf the money wasted on the poll. So, how do you tell good surveys from bad? Good sur-


Surveys


veys are, in a word, scientific. Most people understand that a good scientific survey is based on a representative sample of the population it is intended to represent. But there is one more critical ingredient for proper polling that far fewer are aware of: weights.


Weighted Surveys Are Accurate Surveys Weights determine how much each person in a sample counts. They are re- quired because real-world surveys differ


from the simplified ideal taught in Sta- tistics 101.


In the ideal world of introductory sta- tistics classes, surveys use simple random samples to select respondents. In this ideal,


a survey sample represents the population be- cause each member of the population has an equal probability of being included in it, and each person sampled represents an equal slice of the population. If you sample 1,000 people in a state with a population of ten million, the reasoning goes, each respondent represents 10,000 people. In the real world, surveys of the general population of the United States and most other countries never use simple random samples where everyone has an equal chance of inclusion, because there is no comprehensive list of all the people in the population from which a poll-


28 Campaigns & Elections | Canadian Edition


are not all created equal. A good survey reveals impor-


Illustrations by Keith Bishop for Pitch Graphics/istock


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