EXPERIMENTAL RESEARCH DESIGN

Chapter 8

Research Design

The plan or strategy used to answer the research question

This is where we put to use all of the things that we have learned about validity and reliability.

Experimental Research Design• Weak vs. Strong experimental design

Weak Experimental Designs

Do not control for many extraneous variables Provide weak evidence of cause and effect Have threats to internal validity

One-group Posttest- Only Design

Influence of a treatment is investigated with only one group of individualsSingle test after treatment (“posttest”)

Example: New reading program in 1st gradeDoes program improve reading skills?

One-group Posttest-only Design

Treatment Posttest Measure

X O

Problems? Extraneous variables? Why/when would you use this design? If there is background information on DV and prior

work has shown mechanism by which IV impact DV.

One-group Pretest-Posttest Design

A treatment condition is interjected between pre- and posttest of the dependent variable.

Pretest measure Treatment Posttest Measure

O X O

Compare

One-group Pretest-Posttest Design

Advantage over one-group posttest? Problems? Threats to internal validity? Why would we use this design?

If we cannot create an equal group for comparison, we can get SOME information this way

Nonequivalent Posttest-Only Design

Performance of an experimental group is compared with that of a nonequivalent control group at posttest

TreatmentPosttest Measure

Experimental Group X O

Control Group OCompare

Advantage over previous designs? Problems? Threat to internal validity?

Selection Best thing to do is to use random assignment.

Creates equivalent groups Next best is to use matching on relevant variables.

Will discuss this next chapter

Strong Experimental Research Designs Designs that effectively control extraneous

variables and provide strong evidence of cause and effect

To control for extraneous variables you must eliminate potential rival hypotheses.

Done in two ways:Control Techniques- more on this next week (ch. 7)Control Group

Control groupcomparison groupDoes not receive “active” level of IV

○ No intervention or standard experience

Experimental groupreceives a level of the IV intended to produce

effect

Functions of a Control Group

1. Serve as a comparison to the experimental group. Did the treatment/manipulation have an effect?

Estimate Counterfactual- what the participants’ responses would have been had they not received the treatment

Functions of a Control Group2. Control for rival hypotheses

With random assignment, extraneous variables will have an equivalent impact on both groups

Strong Experimental Research Designs Basic designs – one IV and one DV

Between-participantsWithin-Participants (repeated measures)

Factorial Designs – multiple IVs

Posttest-Only Control Group Design

This design looks familiar, right? What is different now?

TreatmentPosttest Measure

Experimental Group X O

Control Group OCompare

Posttest-Only Control Group Design

We could have more than 1 experimental group

TreatmentPosttest Measure

Control Group O

Experimental Group 1 X1 O

Experimental Group 2 X2 OCompare

Important time-out:

Benefits of randomized control group Avoid many threats to internal validity

Examples?

Pretest-Posttest Control Group Design Simply add pretest to previous design What comparisons will we make?

PretestMeasure Treatment Posttest

Measure

Experimental Group O X O

Control Group O O

Benefits of Pretest

Ensure equivalency of groups Detect ceiling and floor effects

Select participants accordingly Can empirically demonstrate effect of treatment

Change scores See if initial position on DV is important

Treatment may affect hi/low differently