1.1. Choice of parameter
Most ISV research uses measures of variance, such as standard deviation (SD) or mean absolute deviation (MAD). These parameters measure the spread of observations around the mean, irrespective of the direction of the deviation. RT distributions, however, are not Gaussian and show significant rightward skew. It is thus questionable whether such measures of ISV are appropriate, or whether they make an implicit but erroneous assumption of Gaussianicity.
Indeed there is evidence suggesting that this asymmetry, characterised by an extended tail at the slow end of the RT distribution, may be important. Larson and Alderton (1990) coined the worst performance rule by showing that slow RTs are more highly correlated with IQ than fast RTs. These right-tail RTs also discriminate between children with and without ADHD (Leth-Steensen, KingElbaz, & Douglas, 2000), and thus may be of special interest to differential psychologists.