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Randomisation James Erskine

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Randomisation n Youre so random, compliment or insult? n Randomisation is a mathematical / statistical concept. It means there is no detectable systematicity in an event sequence. Thus, random refers to the lack of structure or regularity. Sometimes there is no regularity in behaviour hence - Youre so random.

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Types of Randomisation n Random sampling - this is distinct from random allocation to experimental groups but often confused. This may or may not occur in experiments, most commonly it does not. n Random allocation to experimental conditions. This is vital for a study to be defined as a true experiment. n Random ordering of experimental stimuli

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Random? How? n Random sampling although nice rarely occurs. Why? Because most of our samples are convenience samples (the ubiquitous undergraduates), where our samples are not of an undergraduate flavour they tend to be clinical groups we have specifically targeted (depressed, anxious and so on). Neither are very random.

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How do we become more random? n Random allocation to groups is crucial. n Make sure there is no systematicity in the method you allocate participants to groups by. Alternating between your two experimental groups is clearly not random. Putting 1 and 2 on bits of paper and picking them out of a hat (yes it can be a bowl too) is one method of becoming random. Or for the more sophisticated we could use random number tables in the back of most statistics books.

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How do we become more random continued 2 n How do we use random number tables? Here we just pick a number to start, lets say 7, then go to line 7 and read the numbers from left to right, each time you come across a 1 or 2 you allocate a participant to that group, e.g. if the first two numbers of line 7 are both 1 then participants 1 and 2 are both in group 1. Strictly speaking the number 7 should also be chosen randomly.For a web based random number generator go to:

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Randomness within experimental tasks n Experimental stimuli should also be random. E.g. if you are creating a lexical decision task stimuli should be randomly dispersed throughout the task. However this will often not be possible depending on the nature of your investigation. If a fixed order must be used introduce as much randomness as possible.

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Conclusions n For experimenters being random is a great compliment. n We should all strive to be more random in our experiments.

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Useful Links n n trochim.human.cornell.edu/kb/random.htm

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