Experiment Basics Designs Psych 231 Research Methods in
Experiment Basics: Designs Psych 231: Research Methods in Psychology
Exam 2 on Tuesday n First draft of Class Experiment APA paper due in Labs this week n Review sessions after labs in DEG 13 n Announcements
n Advantages n Interaction effects – Can’t “see” interaction effects without factorial design – Consider the interaction effects before trying to interpret the main effects – Adding factors decreases the variability – Because you are controlling more of the variables that influence the dependent variable – This increases the statistical Power (your ability to detect an effect) of the statistical tests – Increases generalizability of the results – Because you have a situation closer to the real world (where all sorts of variables are interacting) Factorial Designs
n Disadvantages n n n Experiments become very large, and unwieldy The statistical analyses get much more complex Interpretation of the results can get hard • In particular for higher-order interactions • Higher-order interactions (when you have more than two interactions, e. g. , ABC). Action Movie Romantic Comedy Here there is a three way interaction: gender X who stars X type of movie Factorial Designs
n n So far we’ve covered a lot of the about details experiments generally Now let’s consider some specific experimental designs. n n Some bad (but common) designs Some good designs • • 1 Factor, two levels 1 Factor, multi-levels Factorial (more than 1 factor) Between & within factors Experimental designs
n What is the effect of presenting words in color on memory for those words? n So you present lists of words for recall either in color or in black-and-white. Clock Chair Cab n Clock Chair Cab Two different designs to examine this question Example
n Between-Groups Factor § 2 -levels § Each of the participants is in only one level of the IV levels Colored words Clock Chair Cab participants Test BW words Clock Chair Cab
n Within-Groups Factor § Sometimes called “repeated measures” design § 2 -levels, All of the participants are in both levels of the IV levels participants Colored words Clock Chair Cab Test BW words Clock Chair Cab Test
n Between-subjects designs n Each participant participates in one and only one condition of the experiment. n Within-subjects designs n All participants participate in all of the conditions of the experiment. Colored words Test participants BW words participants Colored words Test BW words Test Between vs. Within Subjects Designs
n Between-subjects designs n Each participant participates in one and only one condition of the experiment. n Within-subjects designs n All participants participate in all of the conditions of the experiment. Colored words Test participants BW words participants Colored words Test BW words Test Between vs. Within Subjects Designs
n Advantages: n participants Colored words Test BW words Don’t have to worry about individual differences • Same people in all of the conditions • Variability between conditions is smaller (statistical advantage) n Test Fewer participants are required Within subjects designs
n Disadvantages: n n Range effects Order effects: participants Colored words Test BW words Test • Carry-over effects • Progressive error • Counterbalancing is probably necessary to address these order effects Within subjects designs
n Range effects (context effects) n n 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. $50 $60 $75 $100 Which bike to buy? • Buyers prefer the $60 bike • Add the high end bike -> buyers prefer the $75 bike Within subjects designs
n Carry-over effects n n 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. Condition 1 Condition 2 test Order effects How long do we wait for the effects to wear off? test
n Progressive error n n Practice effects – improvement due to repeated practice Fatigue effects – performance deteriorates as participants get bored, tired, distracted Order effects
n Counterbalancing is probably necessary n 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 ). n All counterbalancing assumes Symmetrical Transfer • The assumption that AB and BA have reverse effects and thus cancel out in a counterbalanced design Dealing with Order Effects
n Simple case n n Two conditions A & B Two counterbalanced orders: • AB • BA Colored words Test BW words Test Colored words Test participants Counterbalancing
n Often it is not practical to use every possible ordering n 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 Example: consider four conditions (Recall: ABCD = 4! = 24 possible orders) 1) Unbalanced Latin square: each condition appears in each position (4 orders) Order 1 A Order 2 B B C D A Order 3 C D A B Order 4 D A B C Partial counterbalancing
n Often it is not practical to use every possible ordering n 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 Example: consider four conditions (Recall: ABCD = 4! = 24 possible orders) 2) Balanced Latin square: each condition appears before and after all others (8 orders) A B C D A B D C B C D A B C A D C D A B C D B A D A B C D A C B Partial counterbalancing
n Between-subjects designs n Each participant participates in one and only one condition of the experiment. n Within-subjects designs n All participants participate in all of the conditions of the experiment. Colored words Test participants BW words participants Colored words Test BW words Test Between vs. Within Subjects Designs
n Clock Colored words Chair Cab Advantages: Test participants BW Clock words Chair Cab n Independence of groups (levels of the IV) • No range effects • Exposure to different levels of the independent variable(s) cannot “contaminate” the dependent variable • No order effects to worry about • Counterbalancing is not required • Sometimes between groups is a ‘must, ’ because you can’t reverse the effects of prior exposure to other levels of the IV • Reduced demand characteristics • Harder to guess what the experiment is about without experiencing the other levels of IV Between subjects designs
n Clock Colored words Chair Cab Disadvantages Test participants BW Clock words Chair Cab n Individual differences between the people in the groups • Excessive variability • Non-Equivalent groups Between subjects designs
n The groups are composed of different individuals participants Colored words BW words Individual differences Test
n The groups are composed of different individuals participants n Colored words BW words Excessive variability due to individual differences n Test Harder to detect the effect of the IV if there is one Individual differences NR R R
n The groups are composed of different individuals participants n Colored words Test BW words Non-Equivalent groups (possible confound) n The groups may differ not only because of the IV, but also because the groups are composed of different individuals Individual differences
n Strive for Equivalent groups n Created equally • Use the same process to create both groups n Treated equally • Keep the experience as similar as possible for the two groups n 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 Dealing with Individual Differences
Group A Red Short 21 yrs Blue tall 23 yrs Green average 22 yrs Brown tall 22 yrs Group B matched Red Short 21 yrs Blue tall 23 yrs Green average 22 yrs Brown tall 22 yrs n Matched groups n n Trying to create equivalent groups Also trying to reduce some of the overall variability • Eliminating variability from the variables that you matched people on Color Height Age Identical twin studies are attempts to do “super” matched group designs Matching groups
n Mixed factorial designs n n Treat some factors as within-subjects (participants get all levels of that factor) and others as between-subjects (each level of this factor gets a different group of participants). This only works with factorial (multi-factor) designs Mixed factorial designs
n n Relevant stuff from Ex 1 Variables n n n types, operationalizing IV: methods of manipulation, getting the right range DV: measurement • Validity and Reliability n n n Sampling Control, Bias, and Confounding Experimental Designs n n Vocabulary Single factor designs Between & Within Factorial designs Exam 2 Topics (Chpts 5, 6, 11, 12)
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