Psychology is the study of human behaviour, which can be done with two types of measures: quantitative and qualitative. 

The field of cognitive psychology examines the mental processes involved when people perform certain tasks with two quantitative measures, namely accuracy and reaction time.

Both measures provide information regarding the relative complexity of the processing of various tasks, based on the assumption that the more difficult tasks will lead to more errors and/or longer reaction times than the simpler ones.

Cognitive psychology has been very useful in understanding some of the processes involved in reading and reading development. For example, research has shown that, especially in children, regular words, which display straightforward correspondences between letters (graphemes) and sounds (phonemes), induce less errors and shorter response times than irregular words, which display inconsistent correspondences.

Obviously, as dyslexia is a condition which induces difficulties with the written language (reading, spelling and writing), people with dyslexia are less accurate and slower than non-dyslexic people in tasks which involve the written language, at least in languages like English or French with so-called opaque writing systems, in which many words display irregular correspondences between graphemes and phonemes. In so-called transparent writing systems as in Spanish, Italian or German, adults and even children with dyslexia are not necessarily less accurate that adults and children without dyslexia. Nevertheless, people with dyslexia remain slower than people without dyslexia. Therefore, in these languages, accuracy is not a reliable measure to identify learners with dyslexia, whereas reaction time is.

Quantitative measures can also be used at the individual level to situate the performance of a child in a given test compared to her/his peers of the same age. To this end, representative samples of learners from various age groups have been administered the test, and statistical analyses regarding their performances have been carried out. This provides norms against which the performance of a single child can be compared in order to determine whether s/he performs within, above or below children of the same age. Most of the tests used by professionals to diagnose dyslexia are based on norms.

For example, in the Rapid Naming Test (RAN), the participant is usually presented with four colors, letters, digits, or pictures of familiar objects, which are repeated randomly several times on a sheet, and is asked to name each item as fast as possible. Denkla and Rudel (1974) have shown that completion time in this test predicted reading abilities in English readers between five and eleven years of age: the faster the children completed the task, the better they performed in reading tasks. Since then, this relationship has been replicated many times in several languages. Not surprisingly, learners with dyslexia perform much slower in the RAN task compared to learners without dyslexia. Consequently, this task is often used, together with a whole battery of other tests, to diagnose dyslexia.

Psychological testing also encompasses qualitative measures such as questionnaires. These can be used in conjunction with quantitative measures or on their own, depending on the topic studied. They provide information regarding the insights of the participants. Statistical analyses using specific tests can also be performed on qualitative data.

Questionnaires are used to assess a range of conditions, such as for example depression or the existence of an Attention Deficit Disorder, with or without Hyperactivity, in a child (ADH/D). In this case, questionnaires are given to the parents and to the teacher of the child in order to assess behavior at home and in the classroom. The results are used in conjunction with quantitative measures examining the attention span of the child, and other aspects, with a view to providing the most accurate diagnosis.