Control Groups, Placebos & Blind Experiments (College Board AP® Statistics)

Control groups & placebos

What is a control group?

• A control group is a treatment group in an experiment that are purposefully given either

  • an inactive form of the treatment

  • or the pre-existing (baseline) treatment

• Having a control group makes it easier to determine whether the treatment of interest (applied to another group) has any effect

What is a placebo?

• A placebo is a dummy (inactive) treatment that the experimental units believe is an active treatment

  • e.g. telling individuals they are taking a pain-relief drink when in fact it is water

• Control groups can use placebos as inactive forms of treatment

  • though placebos are not always required

    • e.g. when comparing a well-established old treatment to a new and untested treatment

What is the placebo effect?

• In some cases, experimental units may show a response to a placebo

  • This type of response is called the placebo effect

    • e.g. feeing like you are smarter after taking what you thought was an intelligence pill (which turned out to be just a sugar pill)

Single & double-blind experiments

What is a single-blind experiment?

• A single-blind experiment is one in which the participants do not know which treatment they are receiving

  • e.g. participants do not know if they have a placebo pill or a real pill

    • It helps to reduce the placebo effect

    • as it stops participants changing their behavior based on knowing what treatment they are having

• The researchers conducting the experiment know which participants received which treatments

What is a double-blind experiment?

• A double-blind experiment is one in which participants and the researchers (in particular those interacting with participants and measuring their responses) do not know which groups are assigned which treatments

  • This helps to reduce the placebo effect

  • and also helps to reduce any bias introduced by the researchers themselves (consciously or unconsciously)

• There will be an experimental designer (who is not a participant, nor a researcher measuring the responses of the participants) who will know which participants received which treatments

  • After the experiment has been conducted, the results are revealed