In its simplest meaning, a process or study that results in the collection of data, the outcome of which is unknown.

In the statistical literacy thread at Level Eight, experiment has a more specific meaning. Here an experiment is a study in which a researcher attempts to understand the effect that a variable (an explanatory variable) may have on some phenomenon (the response) by controlling the conditions of the study.

In an experiment the researcher controls the conditions by allocating individuals to groups and allocating the value of the explanatory variable to be received by each group. A value of the explanatory variable is called a treatment.

In a well-designed experiment the allocation of subjects to groups is done using randomisation. Randomisation attempts to make the characteristics of each group very similar to each other so that if each group was given the same treatment the groups should respond in a similar way, on average.

Experiments usually have a control group, a group that receives no treatment or receives an existing or established treatment. This allows any differences in the response, on average, between the control group and the other group(s) to be visible.

When the groups are similar in all ways apart from the treatment received, then any observed differences in the response (if large enough) among the groups, on average, is said to be caused by the treatment.


In the 1980s the Physicians’ Health Study investigated whether a low dose of aspirin had an effect on the risk of a first heart attack for males. The study participants, about 22,000 healthy male physicians from the United States, were randomly allocated to receive aspirin or a placebo. About 11,000 were allocated to each group.

This is an experiment because the researchers allocated individuals to two groups and decided that one group would receive a low dose of aspirin and the other group would receive a placebo. The treatments are aspirin and placebo. The response was whether or not the individual had a heart attack during the study period of about five years.

See: causal-relationship claim, placebo, randomisation

Curriculum achievement objectives references
Statistical investigation: Levels 5, (6), 7, 8
Statistical literacy: Level 8