Extension. (If you have time in this lesson or for the beginning of the next lesson)

Collect all the students’ graphs and stick these up around the room. While you are doing this – get students to exchange summaries so they all have a different summary to look at. They then have to read the summary and find the graph on the wall they think it refers to. You could make this a competition to see who can spot their graph first. If there is time – repeat the activity.

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Up close and personal: Survey results

The Gallup Organization states its survey results in a universal, statistically correct format. Using a specific example from a recent survey it conducted, you can see the language it uses to report its results:

“These results are based on telephone interviews with a randomly selected national sample of 1,002 adults, aged 18 years and older, conducted June 9–11, 2006. For results based on this sample, one can say with 95% confidence that the maximum error attributable to sampling and other random effects is ±3 percentage points. In addition to sampling error, question wording and practical difficulties in conducting surveys can introduce error or bias into the findings of public opinion polls.”

The first sentence of the quote refers to how the Gallup Organization collected the data, as well as the size of the sample. As you can guess, precision is related to the sample size, as seen in the section “Calculating precision.”

The second sentence of the quote refers to the precision measurement: How much did Gallup expect these sample results to vary? The fact that Gallup is 95 percent confident means that if this process were repeated a large number of times, in 5 percent of the cases the results would be wrong, just by chance. This inconsistency occurs if the sample selected for the analysis doesn’t represent the population — not due to biased reasons, but due to chance alone.