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Understanding statistics
 





Module 1: Producing Data

3. Sources of data > 3.2 Generating your own data > 3.2.2. Generating data from observations

 vii. Ensuring samples are not biased

If you want your samples to be unbiased, you need to be aware of how you generate your samples. Bias can be introduced because of the interaction between the survey participants or units and the person conducting the survey. Possible sources of error in surveying include:

  • response errors (e.g. people might lie about their age, their weight, how many cigarettes they smoke, how much alcohol they drank last week, and so on);
  • missing data (e.g. you might be unable to contact a subject in your study);
  • the effect that the wording of questions has on responses; and
  • the effect that the interviewer might have on participant responses.

Simple random sampling (SRS) is an attempt to ensure that the samples are more representative. However, mistakes can be made with simple random sampling, as the following story shows [4].


Scenario

The Literary Digest, a weekly news magazine in the US, sent mail surveys to 10 million households to develop its opinion poll for the 1936 presidential election. The addresses were obtained from databases of car owners and households with telephones. As a result of its surveys, the Digest predicted the election would be an overwhelming victory for the Republican candidate, Alfred Landon. However, Democrat candidate, Franklin Roosevelt won in a landslide.

Test your knowledge


Question

Which of the following scenarios could have affected the result of the survey?

a) In the 1930's only middle and upper class people could afford a car.

b) Franklin Roosevelt was very unpopular with the middle and upper classes.

c) The response rate was less than 25

d) The people did not like being surveyed
Answer

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Scenario

Below is an account of the process that another newspaper used to obtain a representative sample for a poll it conducted [5]. As you read this account, think about the effort involved .

The poll was based on telephone interviews conducted over 3 days in all parts of the city. A sample of 900 adults was achieved. The interviews were conducted in the two most common languages spoken in households. The sample of telephone exchanges called was selected by computer from a complete list of city exchanges. The exchanges were chosen to ensure that each area in the city was represented in proportion to its population. For each exchange, the telephone numbers were formed by random digits, which ensured that both listed and unlisted numbers were selected. These numbers were screened to ensure that only households (not businesses) were chosen. The results were weighted to take into account household size and the number of telephones in the household, and also to adjust for variations relating to factors such as suburb, sex, age, education and race.






Test your knowledge

Question

Match each of the actions undertaken by the newspaper to how that action made the sample reliable.

Answer

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