a typical experiment

	prototype of a controlled laboratory investigation

In the most typical experiment

	Subjects are randomly assigned to a “treatment” or “control” group.
	The treatment group is exposed to the experimental stimulus.
	The control group is not exposed to the experimental stimulus
	The researcher measures any resulting differences between the treatment and control group

Example 1: treatment group versus control group

	RQ: Does caffeine consumption increase physical stamina?
	Independent variable: caffeine consumption versus no caffeine consumption
	Dependent variable: physical stamina as measured by the total distance jogged on a treadmill in 1 hour.

Random assignment

	Random assignment produces equivalent groups for comparison purposes
	200 volunteers are randomly assigned to a treatment group or control group

Operationalization of the independent variable

	treatment group
		Consumes 12 oz. (500 milligrams) of coffee just before jogging on a treadmill
	control group
		Consumes 12 oz. Of decaffeinated coffee just before jogging on a treadmill

Measurement of the dependent variable

	The total distance of each jogger is measured by the treadmill’s counter to the nearest 1/10 mile.  There is a 1 hr. time limit.  Two observers watch to ensure the joggers are jogging (8-12 minute pace), not walking, until they stop.
	The average distance of the joggers in the treatment group is compared with the average distance of joggers in the control group.

Example 2: comparison groups

	RQ: Do dog owners report greater relationship satisfaction than cat owners?
	Independent variable: pet ownership, e.g., dogs owners versus cat owners
	Dependent variable: relationship satisfaction, as measured by the “Dyadic Adjustment Scale.”

Non-random assignment

	Some experiments rely on “intact” or “self-selected” groups
	People already own their pets. They can’t be randomly assigned to own one pet or another

Operationalization of the independent variable

	91 dog owners are selected using a “mall intercept” technique
	The dog owners do not own a cat or any other pets.
	103 cat owners are selected using a “mall intercept” technique
	The cat owners do not own a dog or any other pets.

Measurement of the dependent variable

	All of the respondents are asked to complete the “Dyadic Adjustment Scale”
		a paper-pencil survey that asks respondents to rate the quality of their relationship with a significant other.
		The average ratings of the dog owners are then compared with the average ratings of the cat owners to see if there is a significant difference.