Experimental Design

What is an experiment? – When the researcher manipulates the independent variable to view change in the dependent variable Why do we do experiments? – To establish and study cause and effect relationships

Famous Experiments Alexander Fleming-Discovery of Penicillen – Initially an accident – But he had to experimentally test its influence to be sure How would you test if a “mold” killed bacteria? How could you make sure something else didn’t kill the bacteria?

Experimental Design-Example Experimental Group Bacteria Control Group Bacteria

Experimental Design-Example Experimental Group Bacteria Control Group Bacteria Mold

Experimental Design-Example Experimental Group Bacteria Dies Control Group Bacteria Control Group Bacteria Lives

How can we be sure of cause and effect? The bacteria died How do we know it was the mold? – Not environment (cold, hot, wind) – Not some other contaminant – Not time/age – Not the shape of the dish – Not a preexisting problem with our bacteria

How can we be sure? We control the circumstances We have a “control group” that is exposed to everything in exactly the same way…except our experimental intervention A true experiment is when the researcher has complete control over who gets the intervention and who does not

Internal Validity When we have high control over the circumstances and outcome of an experiment, we say that it has “good internal validity” Internal validity is the degree to which we can be sure the IV influenced the DV – And we ‘know’ that nothing else caused our outcome

Internal Validity Can be hard to achieve in psychology experiments There are many specific “threats” to internal validity There are ways to deal with some of these threats

Experimental Design-Example The researcher Blue School Green School The researcher wants to know if a special reading program will improve reading scores

Experimental Design-Example The researcher Blue School Green School The researcher wants to know if a special reading program will improve reading scores

How will I know? If reading scores improve, how will I know that its my program? – Can I be sure its not something else? What are some threats to knowing that my program is the cause of the improved reading scores?

Threats to Internal Validity-History History – When an event or circumstance occurs that influences the outcome – What if there was a big “get your kid to read” advertising program in the blue school neighborhood? – What if the green school lost their library funding?

How to “control” history? Can be quite difficult You can ask participants to avoid other treatments… – Though this won’t control all exposures You try to collect good, comprehensive data and remain aware of the circumstances of your participants as much as possible – At least then you can statistically control for some influences in your data

Threats…Maturation Maturation – When growth may account for our effect – This could certainly happen with reading

How to control maturation? You can’t control it But you have a control group Both the experiment and control group should mature at the same rate So any difference would reflect your intervention

Threats--Selection When the groups are unequal in a way that influences outcome – What if my blue schools kids come from poorer neighborhoods than my green schools? – Wealth/Poverty can influence reading development – Poorer kids may start off with lower scores Making it look like my program works

How to control for selection? Randomization – If I have many poor and wealthy schools, and randomly assign, groups should be pretty equal But sample size is crucial here Stratification may help Controlling for Baseline – I can measure reading before intervention – Adjust for existing differences between blue and green schools – Remaining difference should be due to my program

Threats-Participant expectations When participants believe they are getting a treatment So they improve Also known as placebo effect

Controlling placebo? When possible, have a “placebo control” – Control group gets a “treatment” – Gives them attention like intervention group – That should not influence the outcome Control and Intervention should not know which group they are in – If possible…