Research Methods in Psychology STATS 241 Lecture 4 DATE Repeated Measures Designs Incomplete and Complete Definition of Repeated Measures Design Researcher s point of view the same set of subjects serves at all levels of the IV Each subject is measured at each level of the IV Subject s point of view each subject experiences all levels of the IV The ultimate Matched Groups Design Example Mirror drawing task with 3 sided triangle and 4 sided square figures You recycle your subjects over and over again Doing the task repeatedly The person is the same person throughout the study Example Mirror Drawing Task Hypothesis as of changes in direction increases errors will increase Use two figures Triangle and Square 3 sided figure triangle two changes of direction 4 sided figure square three changes of direction Prediction More errors when drawing Square fewer errors when drawing Triangle IV number of changes in each direction DV number of errors or time it takes to complete the shape Suppose we had all participants do first Square and then Triangle and we found fewer errors when tracing the Triangle Could we claim that the lower errors for the Triangle was because it had fewer changes of direction What other plausible explanations might exist The participant could be used to the experiment Practice effect Suppose we had all participants do first Triangle and then Square and we found more errors when tracing the Square Could we claim this was because there are more changes of direction for the square What other plausible explanations might exist Could be something else could be fatigue at this point of the experiment have to worry about these such things alternative explanations but there are ways to deal with that Repeated measures designs are very powerful Perfectly matched subjects at each level of IV Because subjects are measured repeatedly there are potential issues of co varying time related secondary variables threats to internal validity commonly called order effects or carry over effects The most common order effects are practice boredom and fatigue You cannot eliminate these order effects but you can balance them across levels of your IV counterbalancing Counterbalancing in the Stroop task remember this This can be done in several different ways and how you do this is tied to the type name of repeated measures design you use Incomplete versus Complete Incomplete Repeated Measures Design The same set of subjects is measured at all levels of the IV repeated measures but each subject experiences and is measured at each level of the IV only once only one trial at each level of the IV Experiences one time and one time only All levels of the independent variable but you only do the task once at each level Example The Mirror drawing study we just discussed Each person traced a square once and a triangle once DV errors IV type of form traced Levels of IV square triangle Each subject contributed two scores one for square and one for triangle How can we deal with time related secondary variables such as boredom practice fatigue Here is where counterbalancing may come in Counterbalancing for Incomplete Repeated Measures Designs Controlling time related variables that are potential threats to internal validity in incomplete repeated measures designs is easy for a two level IV There are only two possible orders for your IV AB and BA subjects do one order and do the other Does not eliminate order effects but does balance them across the two levels of the IV Stroop Task Lab the order of stimuli read NL INC INC NL 3 Or 3 factorial 3 X 2 X 1 6 With a 3 level IV A B C there are six possible orders ABC ACB BAC BCA CAB CBA Four level IV 4 Orders 4X3X2X1 24 orders Divide subjects into 1 24th For an IV with 3 levels you can use a Latin Square It will make life easier Latin Square an arrangement of symbols in rows and columns such that each 1 6 of subjects for each order symbol occurs only once in each row and each column Each letter occurs once and only once in each column and each row Have 24 subjects of subjects assigned to each order row ACBD 1 4 subjects 6 DACB 1 4 subjects 6 BDAC 1 4 subjects 6 CBDA 1 4 subjects 6 Example of NOT A LATIN SQUARE ACBD DBAC CDBA BADC Why not Third column B is repeated fourth column C is repeated You cannot repeat any letters in the rows or columns How to use a Latin Square 1 Three level IV Levels A B C 2 Equal of subjects for each order row 3 ACB 1 3 of subjects BAC 1 3 of subjects CBA 1 3 of subjects 4 Each letter occurs only once in each row each subject experiences each condition only once 5 Each letter occurs only once in each column balances order effects across levels of IV Example of an Incomplete Repeated Measures Design Effect of exercise on mood Hansen Stevens Coast 2001 page 236 hardback Four levels of exercise 0 30 minutes of quiet resting 10 20 30 min exercise on stationary bike All participants did all levels one time each at one week intervals over a 4 week period Rotation Method of generating a Latin Square Quick dirty not the best method Better method Diagram Balanced Latin Square For this course we will use simple rotation methods Creating a Latin Square 4 level IV Rotation Method 1 Choose a random arrangement of symbols for your conditions for your first row 2 Rotate systematically for other rows For next row put B at far right and slide rest of symbols over one to the left Now put C at far right and slide rest of symbols one to the left For last row put A at far right and slide rest of symbols one to the left First Row BCAD CADB ADBC DBCA Complete Repeated Measures Design Definition The same set of subjects is measured at all levels of the IV repeated measures but each subject experiences and is measured at each level of the IV more than once Counterbalancing Complete Repeated Measures Designs Block Randomization In block randomization each block involves one occurrence of each level of the IV The order of the levels in each block is randomly arranged hence block randomization Four level IV level A level B level C level D Suppose we want 6 trials at each level Generic example of a Block Randomization 4 level IV Each block a random order of the 4 levels for example BDAC could be one block We would need six blocks so each subject experiences each level six times BDAC DACB BADC CADB ACBD DBCA Each subject would be exposed to all 6 blocks Each subject would experience each level A B C D six times Example Sackheim Gur and Saucy 1978 Page 230 233 of