Experimental Design

P291 Research Methods

November 20, 2012

Well designed experiments have:

1. Research hypotheses with specifically predicted causal effects

2. At least 2 levels of one IV

3. Random assignment of participants

4. Procedure for empirically testing the hypothesized causal relationship

5. Specific controls to reduce threats to internal validity

Random Sampling

• Procedure for selecting participants in which each participant has an equal chance of being selected and the selection of any one participant does not affect the probability of selecting another– Random number generator– Coin flipping– Rolling a die

Conditions

• Experimental Conditions– At least 1 IV with at least 2 levels–Manipulation

• Control Conditions– Baseline or non-manipulated level of IV–Manipulation check

Between Subjects Design (Post-Test only Design)

A psychologist wanted to examine the effects of diet on learning in rats. Newborn rats were randomly assigned to one of two diets: standard Purina Rat Chow or a supplemented diet where vitamins and minerals were added to the Rat Chow. After six months, each rat was tested in the Radial Arm Task, and the researcher recorded the number of errors each animal made before it solved the problem.

What is the IV?What is the DV?How do you know this is a Post-test only design?

Randomized Pre Test – Post Test Design

A psychologist wanted to examine the effects of diet on learning in rats. Rats were tested in the Radial Arm Task following quarantine. The rats were then randomly assigned to one of two diets: standard Purina Rat Chow or a supplemented diet where vitamins and minerals were added to the Rat Chow. After six months, each rat was retested in the Radial Arm Task, and the researcher recorded the number of errors each animal made before it solved the problem.

What is the IV?What is the DV?How do you know this is a Pre/Post Test design?

An experiment of our own

• You will be randomly assigned (with constraint) to either group 1 or group 2 using a coin toss.

• Based on your condition you will be given a short questionnaire to fill out.

• When everyone has finished filling out their questionnaire we will discuss the rationale.

• Next week we will show you the results.