Experiments vs. Observational StudiesSpecial Topics

Section 7.6

Observational StudyAn observational study does not try to

manipulate the environment (such as by assigning treatments to people), but simply observes the measurements of variables of interest that result from people's free choices.

This kind of study is generally done when a treatment is unethical (for example, smoking while pregnant) and/or impossible (such as ethnicity) to assign to a person.

Double-Blind ExperimentDouble-blind experiment: An experiment

in which neither the experimental subjects nor the persons who interact with them know which treatment each subject received.

When a subject knows what the treatment is, it can add confounding and contaminate the results.

Placebo EffectPlacebo effect: The effect of a dummy

treatment (such as an inert pill in a medical experiment) on the response of subjects.

A subject may respond to any treatment, even a dummy treatment. This is due to the expectation of something good happening as a result of medical intervention, or due to faith in the attending medical practitioner.

There is such a thing as “placebo surgery”!

Statistical SignificanceWhile the results may show a difference

between groups in an experiment, we need to make sure the difference is statistically significant.

Definition: An observed effect so large that it would rarely occur by chance is called statistically significant.

Lack of Realism The novelty of an experimental setting might

not allow conclusions found in the experiment to generalize to real-world situations.

Once the novelty wears off, the effects of a treatment, observed in an experiment, may wear off as well.

HomeworkWorksheet 7-6.