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CHAPTER 7: Repeated Measures Designs 233TABLE 7.1BLOCK-RANDOMIZED SEQUENCE OF 54 TRIALS IN AN EXPERIMENTWITH THREE CONDITIONS ADMINISTERED 18 TIMES EACHTrial Conditions Trial Conditions1 O First28 O2 L Block29 L3 R 30 R4 R 31 R5 O 32 L6 L 33 O7 R 34 O8 O 35 R9 L 36 L10 L 37 L11 O 38 R12 R 39 O13 O 40 O14 L 41 R15 R 42 L16 R 43 R17 L 44 O18 O 45 L19 R 46 R20 L 47 L21 O 48 O22 L 49 O23 R 50 R24 O 51 L25 R 52 R26 L 53 O27 O 54 LNote: The conditions are the three versions of the photographs used bySackeim et al. (1978): L left composite, O original, R right composite.average out. We should not expect practice effects to be balanced after two orthree blocks—any more than we would expect sample sizes of two or threein the random groups design to result in comparable groups. Fortunately,a technique is available to balance practice effects when it is not possible toadminister each condition often enough for the averaging process of block randomizationto work effectively.ABBA Counterbalancing In its simplest form, ABBA counterbalancing can beused to balance practice effects in the complete design with as few as twoadministrations of each condition. ABBA counterbalancing involves presentingthe conditions in one sequence (i.e., A then B) followed by the opposite of that