Chapter 8 Experimental Design
Confounding variables and internal validity • The means by which independent variables are kept constant • Confounding variable • A variable that fluctuates along with the independent variable • Confounding occurs when the effects of 2 IVs are intertwined making it unsure of which IV is responsible for the observed effect • Eliminating possible confounds makes for good experimental design • Internal validity • When the results of an experiment can confidently be attributed to the effect of the IV
Find the confounding variables A researcher is interested in the ability of schizophrenic patients to judge different time durations. It is hypothesized that loud noise will negatively affect judgment. Subjects are tested • Quietly in a sound proof room or • Noisily in a nurse’s office w/ a stereo playing music at a loud and constant volume Due to scheduling problems, Risk III patients are available only on Mondays. Risk I patients are available only on Thursdays. Also, hearing tests are scheduled for Thursdays, so the sound proof room is only available on Monday.
Basic experiments • Posttest-only design • Researcher must • Obtain 2 equivalent groups • Introduce the independent variable • Measure the effect of the independent variable on the dependent variable • R = random assignment Experimental group R Measure Participants Control group R Measure
Basic experiments • Posttest-only design • Avoid selection differences where people selected differ in any way • Accomplish equivalence by randomly assigning participants • If groups were equally selected and there were no confounds, any difference in the DV groups must be attributed to the IV
Basic experiments • Pretest-posttest design • Researcher must • Give pretest before experimental manipulation • The larger the sample, the less likely groups will differ before manipulating the independent variable (IV) • R = random assignment R Experimental group Pretest Treatment Posttest Participants Posttest Control group Pretest R
Basic experiments • Advantages • . • . • . • Disadvantages • . • .
Basic experiments • Solomon four-group design • ½ participants = only posttest • ½ participants = pretest and posttest • The effect of the pretest can be directly assessed • Possible outcomes • Results are the same with or without a pretest • A difference b/w the experimental and control groups w/ a pretest BUT no difference with no pretest
Assigning participants • Two methods • Random assignment (independent groups design) • Participants are in all conditions (repeated measures design • Advantages • Needs less participants; Less training of participants • Sensitive to finding statistically significant differences • Individual differences can be seen and explained • Disadvantages • Order effects: the order of presenting the treatments affects the dependent variable. • Performance on the 2nd task may be better than the 1st
Assigning participants • Types of order effects • Practice effect • Fatigue effect • Contrast effect
Should the IV be between- or a within-subjects design? • A neuroscientist hypothesizes that damage to the primary visual cortex is more permanent in older animals • A sensory psychologist predicts that it is easier to distinguish slightly different shadings of grays under daylight than under fluorescent light • A developmental psychologist predicts cultural differences in moral development
Counterbalancing • Used in repeated measures designs • Complete counterbalancing • All possible orders of presentation are included in the experiment • Latin square • A limited set of orders where • Each condition appears at a ordinal position • Each condition precedes and follows each condition at one time
Counterbalancing • Randomized blocks • Each repetition in the basic experiment is called a block of trials • To control for order effects the order of presentation can be randomly determined
Matched pairs design • Match people on a certain characteristic • Rank participants based on scores on the matched variable • Members of each pair are randomly assigned to the conditions in the experiment • Each treatment is given to one subject in each pair • In the two different groups one of which will act as the control, the other as the experimental. • Ensures that groups are equivalent • Drawbacks • Time consuming • Needs a large sample pool
Developmental research designs • Cross sectional method • Persons of different ages are studied at only one point in time • Developmental change is not observed but based on comparisons of different cohorts • More common and yields immediately useful results • Longitudinal method • The same group of people is observed at different time points in time as they grow older • Over the course of the study people may move, die, drop out… • Sequential method • Combines both methods • First phase is cross sectional then they are studied longitudinally • Two or more age cohorts are tested at two or more times.