Psychometric Tests
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What is a constant?
A value that does not differ

What is a variable?
A value that differs across individuals or conditions

What is a discrete variable?
A variable with a finite (or infinite if countable) range of values

What is a dichotomous variable?
 A type of discrete variable
 Can only assume 2 values (e.g. coin sides)

What is a Polytomous variable
 A type of discrete variable
 More than 2 values

What are continuous variables?
 Variables with no theoretical limit
 Measured on scales which units can be infinitely subdivided
 Because of this we never get the true value, only an estimate

What are nominal scales? Stevens (1946)
 Numbers used as labels to identify a class so most used with categorical data (e.g. females=1, males=2)
 All members of a particular set must be equal regardless of characteristics
 The numbers are only there to delineate categories so there is no point using them for stats tests apart from ratios using numbers in each category

What are ordinal scales? Stevens (1946)
 Same as nominal but also specify order
 No indication of difference between ranks, only their order
 Chiefly used for percentile scores
 Can be manipulated the same as normal data and non parametric tests can be carried out

How do you convert a raw score into a percentile score?
 Score sample mean over sample standard deviation
 This gives us Z scores
 Z scores can be looked up in a distribution table

What are interval scales? Stevens (1946)
 The difference between two numbers reflects an equal distance in what is being measured
 The 0 is arbitrary (0 for temp doesn't mean no temp)
 This means ratios cannot be calculated as there is no true 0

What is a ratio scale? Stevens (1946)
 True zero and equal intervals
 Has the mathematical properties of identity, magnitude, equal intervals, and a true zero.
 Used when measuring frequency of behaviour, time intervals and number of correct responses

What are the limitations of ordinal scales?
 Many scores are reported as %
 Disrrivution in ranks is always greater at the extremes
 If you compared the 40th and 45th percentile the difference would be different than for the 90th and 95th

What are the limitations of ratio IQ?
 StanfordBinet intelligence scale
 Ratio is calculate by dividing mental age on the SBIS with the actual age and dividing by 100
 There are issues as chronological age rises steadily on a ratio scale, however mental age is ordinal