Evidence-Based Management

Figure 3.1

The magazine used a biased sampling plan. They selected the sample using magazine subscriptions, lists of registered car owners and telephone directories. But that sample was not representative of the US public. In the 1930s, Democrats were much less likely to own a car or have a telephone. The sample therefore systematically underrepresented Democrats, and the result was a whopping error of 19 per cent, the largest ever in a major public opinion poll.²

You can prevent selection bias by taking a random sample of the population. However, to get a truly random sample you should not only consider who to ask, but also when to ask. ...