A definition on this basis could be expressed in the following way:
A reading achievement which is clearly below that which can be expected in relation to persons of the same age, is considered as dyslexic.
Such definitions consider neither intelligence nor other factors/situations which may explain weak performances in reading and writing. They presuppose that it is known what, at any one time, are normal reading and writing skills. These definitions have also been criticised, and one reason is that all types of reading difficulties become encompassed by the concept of dyslexia. Additionally, there is no agreement on how wide the discrepancy from the expected reading skills should be, before it is considered dyslexia.
Other definitions of dyslexia specify difficulties and include causes.