Experimental Control cont. Psych 231: Research Methods in Psychology

Announcements Re-writes of group project intros due this week in lab –Please attach the original intro draft Group Project Methods section due next week in lab –Please bring your textbook to lab Remember to download and read the articles for class exp soon (e.g., before next week’s labs)

Sources of variability (noise) Sources of Total (T) Variability: T = NR exp + NR other +R Our goal is to reduce R and NR other so that we can detect NR exp. That is, so we can see the changes in the DV that are due to the changes in the independent variable(s).

Weight analogy Imagine the different sources of variability as weights R NR exp NR other R NR other Treatment groupcontrol group

Weight analogy If NR other and R are large relative to NR exp then detecting a difference may be difficult R NR exp NR other R NR other

Weight analogy But if we reduce the size of NR other and R relative to NR exp then detecting gets easier R NR other R NR exp NR other

Using control to reduce problems Potential Problems –Excessive random variability –Confounding –Dissimulation

Potential Problems Excessive random variability –If control procedures are not applied, then R component of data will be excessively large, and may make NR undetectable –So try to minimize this by using good measures of DV, good manipulations of IV, etc.

Excessive random variability R NR exp NR other NR other R Hard to detect the effect of NR exp

Potential Problems Confounding –If relevant EV co-varies with IV, then NR component of data will be "significantly" large, and may lead to misattribution of effect to IV IV DV EV Co-vary together

Confounding R NR exp NR other Hard to detect the effect of NR exp because the effect looks like it could be from NR exp but is really (mostly) due to the NR other R

Potential Problems Potential problem caused by experimental control –Dissimulation If EV which interacts with IV is held constant, then effect of IV is known only for that level of EV, and may lead to overgeneralization of IV effect This is a potential problem that affects the external validity

Methods of Controlling Variability Comparison Production Constancy/Randomization

Methods of Controlling Variability Comparison –An experiment always makes a comparison, so it must have at least two groups Sometimes there are control groups This is typically the absence of the treatment –Without control groups if is harder to see what is really happening in the experiment –it is easier to be swayed by plausibility or inappropriate comparisons Sometimes there are just a range of values of the IV

Methods of Controlling Variability Production –The experimenter selects the specific values of the Independent Variables (as opposed to allowing the levels to freely vary as in observational studies) –Need to do this carefully Suppose that you don’t find a difference in the DV across your different groups –Is this because the IV and DV aren’t related? –Or is it because your levels of IV weren’t different enough

Methods of Controlling Variability Constancy/Randomization –If there is a variable that may be related to the DV that you can’t (or don’t want to) manipulate you should either hold it constant (control variable) let it vary randomly across all of the experimental conditions (random variable) –But beware confounds, variables that are related to both the IV and DV but aren’t controlled

Poorly designed experiments Example: Does standing close to somebody cause them to move? –So you stand closely to people and see how long before they move –Problem: no control group to establish the comparison group (this design is sometimes called “one-shot case study design”)

Poorly designed experiments Does a relaxation program decrease the urge to smoke? –One group pretest-posttest design –Pretest desire level – give relaxation program – posttest desire to smoke

Poorly designed experiments One group pretest-posttest design –Problems include: history, maturation, testing, instrument decay, statistical regression, and more participantsPre-test Training group Post-test Measure Independent Variable Dependent Variable

Poorly designed experiments Example: Smoking example again, but with two groups. The subjects get to choose which group (relaxation or no program) to be in Non-equivalent control groups –Problem: selection bias for the two groups, need to do random assignment to groups

Poorly designed experiments Non-equivalent control groups participants Training group No training (Control) group Measure Self Assignment Independent Variable Dependent Variable

“Well designed” experiments Post-test only designs participants Experimental group Control group Measure Random Assignment Independent Variable Dependent Variable

“Well designed” experiments Pretest-posttest design participants Experimental group Control group Measure Random Assignment Independent Variable Dependent Variable Measure Dependent Variable

Next time Read chapters 8 & 10