That excerpt actually comes from the section on Response to Intervention. But in any case, factors which might alter the validity of the discrepancy value would include anything which could have resulted in deflated IQ or achievement scores. Such factors typically include illness, lack of motivation, attentional difficulties, impulsivity, chemical use, inconsistent educational exposure, anxiety, depression, etc., etc. But even if both the IQ and achievement scores are accurate, the discrepancy value really represents a probability that scores at or below that value truly represent skills which are significantly below ability. For example, if the discrepancy value is based on a confidence level of 90%, then 90% of the time a discrepancy of that magnitude is actually significant. But 10% of the time it is not. Of course that also means that there will be many times when a smaller discrepancy between scores actually represents a significant discrepancy between skills and ability. Since there really are no absolutes in testing and measurement, modern assessment is largely based upon probabilities. As such it is very important for examiners to look beyond obtained scores when making diagnostic considerations.