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: The Stroop effect with pictureword stimuli Suppose we had all participants do first “Incongruent label” (INCL) and then “No label” (NL) and we found faster times to name the silhouettes with no label (as predicted). What other “plausible explanations” might exist? What if we did first NL and then INCL and found slower times for INCL? How did we “fix” this issue in our study? • 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 timerelated 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) • 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 but each subject experiences and is measured at each level of the IV only once (only one trial at each level of the IV). • Example: Lab Stroop task. Each person was exposed and measured once with NL and once with INCL. • DV=time to complete one sheet. Each subject contributed two scores, one for NL and one for INCL. • How did we deal with time-related secondary variables such as boredom, practice, fatigue? 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 • ½ subjects do one order and ½ do the other • Does not eliminate order effects but does “balance” them over the two levels of the IV. • With a 3-level IV there are six possible orders (3! Or 3 factorial=3 X 2 X 1=6) • ABC • ACB • BAC • BCA • CAB • CBA • 1/6 of subjects for each order! • Four level IV= 4! Orders=4X3X2X1=24 orders! Divide subjects into 1/24th! • For an IV with >3 levels, can use a “Latin Square” • Latin Square: an arrangement of symbols in rows and columns such that each symbol occurs only once in each row and each column Latin Square Example for a 4-level IV Each letter occurs once and only once in each column and each row. ¼ of subjects assigned to each “order” (row) ACBD DACB BDAC CBDA NOT a Latin Square!!!! ACBD DBAC CDBA BADC How to use a Latin Square • Three level IV: Levels= A, B, C • Equal # of subjects for each order (row). • ACB (1/3 of subjects) BAC (1/3 of subjects) CBA (1/3 of subjects) • Each letter occurs only once in each row=each subject experiences each condition only once. • 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 248 pb, 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 and “dirty”, not the best method • Better method= “Diagram Balanced Latin Square” • Use a random arrangement of symbols for your conditions for your first row. A= 0 min, B=10 min, C=20 min, D=30 min First Row: BCAD • For next row, put B at far right and slide rest of symbols over one to the left CADB • Now put C at far right and slide rest of symbols one to the left ADBC • For last row, put A at far right and slide rest of symbols one to the left DBCA • A= 0 min, B=10 min, C=20 min, D=30 min BCAD (1/4 subjects) CADB (1/4 subjects) ADBC (1/4 subjects) DBCA (1/4 subjects) Complete Repeated Measures Design Definition • The same set of subjects is measured at all levels of the IV but each subject experiences 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”) Generic example of a Block Randomization (4 level IV) • Four level IV: level A, level B, level C, level D. • Suppose we want 6 trials at each level • One block= a random order of the 4 levels (A,B,C,D), for example “BDAC” is one block. • We would need 6 blocks: 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. Research example of block randomization • Sackheim, Gur and Saucy (1978) (Page 232233 of text) • Does one side of our face express emotion more intensely than the other? • Set of photos of people expressing emotions • Cut photos in half down middle and created: – Composite photos of two left sides (L) – Composite photos of two right sides (R) – Original photo (O) – One IV (type of photo) with three levels: O, L, R Three versions of “disgust”:L,O,R • One IV (type of photo) with three levels: O, L, R • Had different people pose expressing several different emotions (disgust, fear, joy etc) • Each participant viewed 54 photos, 18 O, 18L, and 18 R • Participants rated each photo on a 7-point scale indicating the intensity of the emotion expressed. • Photos ordered by “block randomization” • Each block contains one O, one L, and one R. • There were 18 blocks altogether with O,L, R randomly ordered in each block. Which do you think showed most intense emotion? Left side(a), Original (b), or Right side (c)? • Findings: Most people judged left-side composite (L) as showing more intense emotion. We may display stronger emotion with the left side of our face. (controlled by right hemisphere of brain).