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194 PART III: Experimental MethodsKey ConceptA common procedure for carrying out random assignment is block randomization.First, let us describe exactly how block randomization is carried out,and then we will look at what it accomplishes. Suppose we have an experimentwith five conditions (labeled, for convenience, as A, B, C, D, and E). One“block” is made up of a random order of all five conditions:One block of conditions →A B C D ERandom order of conditionsC A E B DIn block randomization, we assign subjects to conditions one block at a time. Inour example with five conditions, five subjects would be needed to completethe first block with one subject in each condition. The next five subjects wouldbe assigned to one of each of the five conditions to complete a second block, andso on. If we want to have 10 subjects in each of five conditions, then there wouldbe 10 blocks in the block-randomized schedule. Each block would consist of arandom arrangement of the five conditions. This procedure is illustrated belowfor the first 11 participants.10 Blocks Participants Condition1) C A E B D 1) Cara → C2) E C D A B 2) Andy → A3) D B E A C 3) Jacob → E First block4) B A C E D 4) Molly → B5) A C E D B 5) Emily → D6) A D E B C 6) Eric → E7) B C A D E 7) Anna → C8) D C A E B 8) Laura → D Second block9) E D B C A 9) Sarah → A10) C E B D A 10) Lisa → B}}11) Tom → Dand so on for 50 participantsThere are several advantages when block randomization is used to randomlyassign subjects to groups. First, block randomization produces groupsthat are of equal size. This is important because the number of observationsin each group affects the reliability of the descriptive statistics for each group,and it is desirable to have the reliability of these measures comparable acrossgroups. Block randomization accomplishes this. Second, block randomizationcontrols for time-related variables. Because experiments often take a substantialamount of time to complete, some participants can be affected by eventsthat occur during the time the experiment is conducted. In block randomization,every condition is tested in each block so these time-related variables arebalanced across the conditions of the experiment. If, for example, a traumaticevent occurs on a college campus in which an experiment is being conducted,the number of participants who experienced the event will be equivalent ineach condition if block randomization is used. We assume, then, that the