Ethical issues arise

There are a few practicalities that complicate experimentation with human subjects. For ethical reasons, experiments involving potential danger to the subjects are not possible. Even if there is no known danger, the subjects should be aware of what is involved in the experiment and must give informed consent.

Controls

There are many instances where an experiment is intended to measure the effect of a particular treatment, such as the improvement of a medical condition caused by administration of a particular drug. A naive experimenter may record the value of some variable (e.g. concentration of some chemical in the blood) before medication is commenced and also after the medication has been used for a week. However any improvement in the condition may have resulted simply from the passage of time and it may not be related to the drug. Time is a lurking variable that is confounded with the treatment.

In order to assess the effect of the treatment, some (randomly selected) subjects who have not received the treatment should also be included in the study — differences between the improvements of the two groups can then be credited to the treatment. Subjects who receive no treatment are called controls.

Placebos

Unfortunately, the act of administering a treatment to a human subject may itself affect the response, irrespective of the treatment effect. For example, if a drug is being assessed for its ability to reduce headaches, the knowledge that medication has been administered may make the subject feel better, even if the drug has no active ingredient.

To avoid the psychological effect of the treatment on the subject being confounded with the effect of the drug, an indistinguishable 'treatment' with no effect may be given to the control group of subjects; this is called a placebo. For example, two batches of pills of similar size and taste may be prepared, with only one batch containing the active ingredient being assessed. Any difference between the control group and the treatment group can therefore be attributed to the treatment.

Double-blind experiments

A further complication may arise when the measured response from each subject may be affected by knowledge of the treatment applied. If the experimenter knows which treatment has been applied to each experimental unit, there may be a subconscious tendency to systematically over- or under-assess one treatment. To avoid this potential problem, the experimenter may be unaware of which experimental units received which treatment until after the experiment.

The experiment is called double-blind if neither the experimenter not the subject knows which treatment has been applied. For example, a third party may randomly decide which of two drugs will be given to each subject, and package the appropriate pills for each subject in unlabelled containers. The experimenter would administer the treatments and record results without knowing which subjects were receiving which treatments. Again, the aim is to ensure that other factors do not act as lurking variables to confound comparisons of the treatments.