Experiment Basics: Designs,Psych 231: Research Methods in Psychology,Experimental designs,So far weve covered a lot of the about details experiments generally Now lets consider some specific experimental designs. Some bad (but common) designs Some good designs 1 Factor, two levels 1 Factor, multi-levels Factorial (more than 1 factor) Between & within factors,Example,What is the effect of presenting words in color on memory for those words?,Two different designs to examine this question,participants,Colored words,BW words,Test,2-levelsEach of the participants is in only one level of the IV,Between-Groups Factor,Clock Chair Cab,Clock Chair Cab,2-levels, All of the participants are in both levels of the IV,Clock Chair Cab,Clock Chair Cab,Sometimes called “repeated measures” design,Within-Groups Factor,Between vs. Within Subjects Designs,Within-subjects designs All participants participate in all of the conditions of the experiment.,Between-subjects designs Each participant participates in one and only one condition of the experiment.,Within-subjects designs All participants participate in all of the conditions of the experiment.,Between-subjects designs Each participant participates in one and only one condition of the experiment.,Between vs. Within Subjects Designs,Between subjects designs,Advantages:Independence of groups (levels of the IV) Harder to guess what the experiment is about without experiencing the other levels of IV Exposure to different levels of the independent variable(s) cannot “contaminate” the dependent variable Sometimes this is a must, because you cant reverse the effects of prior exposure to other levels of the IV No order effects to worry about Counterbalancing is not required,Between subjects designs,DisadvantagesIndividual differences between the people in the groups Excessive variability Non-Equivalent groups,Individual differences,The groups are composed of different individuals,Individual differences,The groups are composed of different individuals,Individual differences,The groups are composed of different individuals,Dealing with Individual Differences,Strive for Equivalent groups Created equally - use the same process to create both groups Treated equally - keep the experience as similar as possible for the two groups Composed of equivalent individuals Random assignment to groups - eliminate bias Matching groups - match each individuals in one group to an individual in the other group on relevant characteristics,Matching groups,Group A,Group B,Matched groups Trying to create equivalent groups Also trying to reduce some of the overall variability Eliminating variability from the variables that you matched people on,Red Short 21yrs,Blue tall 23yrs,Green average 22yrs,Brown tall 22yrs,Color Height Age,Within-subjects designs All participants participate in all of the conditions of the experiment.,Between-subjects designs Each participant participates in one and only one condition of the experiment.,Between vs. Within Subjects Designs,Within subjects designs,Advantages:Dont have to worry about individual differences Same people in all the conditions Variability between conditions is smaller (statistical advantage) Fewer participants are required,Within subjects designs,Disadvantages Range effects Order effects: Carry-over effects Progressive errorCounterbalancing is probably necessary to address these order effects,Within subjects designs,Range effects (context effects) can cause a problem The range of values for your levels may impact performance (typically best performance in middle of range). Since all the participants get the full range of possible values, they may “adapt” their performance (the DV) to this range.,Order effects,Carry-over effects Transfer between conditions is possible Effects may persist from one condition into another e.g. Alcohol vs no alcohol experiment on the effects on hand-eye coordination. Hard to know how long the effects of alcohol may persist.,How long do we wait for the effects to wear off?,Order effects,Progressive error Practice effects improvement due to repeated practice Fatigue effects performance deteriorates as participants get bored, tired, distracted,Dealing with order effects,Counterbalancing is probably necessary This is used to control for “order effects” Ideally, use every possible order (n!, e.g., AB = 2! = 2 orders; ABC = 3! = 6 orders, ABCD = 4! = 24 orders, etc). All counterbalancing assumes Symmetrical Transfer The assumption that AB and BA have reverse effects and thus cancel out in a counterbalanced design,Counterbalancing,Simple case Two conditions A & B Two counterbalanced orders: AB BA,Counterbalancing,Often it is not practical to use every possible ordering Partial counterbalancing Latin square designs a form of partial counterbalancing, so that each group of trials occur in each position an equal number of times,Partial counterbalancing,Example: consider four conditions Recall: ABCD = 4! = 24 possible orders 1) Unbalanced Latin square: each condition appears in each position (4 orders),