From experiments
Some contingency tables arise from experiments.
Vitamin C and colds
From surveys
Surveys are conducted to ascertain voting intentions, purchases of consumer goods, satisfaction with courses, and for a variety of other research purposes. The next chapter will discuss general principles of data collection from surveys.
Individuals from some target group are usually given a questionnaire to complete. The individual questions are often answered by ticking boxes (e.g. 'Approve', 'Neutral' or 'Disapprove') and are therefore categorical. Some of the resulting categorical variables can often be considered to split the respondents into groups.
Survey data are often reported using many contingency tables.
Contraception and sexual health