The adaptive algorithms in LADS Plus
All five of the modules in LADS Plus are adaptive, which makes screening swift and effective. However, the adaptive algorithms6 that have been employed differ between the tests. The Working Memory module adapts itself to performance of the person taking the test simply by discontinuing after a both items at a given difficulty level have been failed. The reasoning modules use probe items to identify the section of the test that is most appropriate for assessing each individual and then the program administers items that are easier or harder, according to individual performance, discontinuing the test when that person’s ceiling has been reached.
The other two modules (Word Recognition and Word Construction) employ a technique generally known as CAST (Computerised Adaptive Sequential Testing), in which blocks of items of known difficulty are administered in an adaptive sequence (see Drasgow and OlsonBuchanan, 1999). In these two LADS Plus modules, the CAST approaches utilises an fractionation algorithm that assign persons being assessed to a category, based on their performance on each module. The categories used are as follows:
In other words, the higher the score on each assessment module in LADS Plus, the higher the probability that the person has dyslexia. The adaptive fractionation algorithm operates by giving the person blocks of assessment items of similar difficulty and then applying decision rules to the outcome. These decision rules determine whether the individual either (a) clearly falls into one of the specified categories, or (b) whether more blocks of items of a different level of difficulty should be administered before re-applying the decision rules. The difficulty level of each item has already been determined by trials involving both known dyslexic and non-dyslexic adults. This method is called algorithmic fractionation because of an analogy with chemical fractionation, in which a compound mixture is separated into its various components, usually by subjecting it to different temperatures so that different components vaporise and may be condensed out. In LADS Plus, blocks of assessment items of different difficulty are administered, and the person’s response to these items enables the program to make a separation into the designated categories.
6 An ‘algorithm’ is the name given to any machine-like procedure that is carried out in a sequence of steps and which guarantees a solution to a problem. [As opposed to a ‘heuristic’, a method involving trial-and-error learning, which, although often quicker, may or may not result in a solution to a problem.]