There is still a lot of confusion concerning the concept of deafness. Who are the deaf? How many are they? To have precise and official data is almost impossible, principally because only a doctor can determine the degree of deafness in a subject.
Deafness is a more or less heavy reduction of the auditory faculty. Many factors have to be taken into consideration, which are all closely connected to each other: how old was the person when the hearing reduction occurred, the actual decibels lost, and the re-educational linguistic method that has been adopted.
Regarding age, it is very important to understand whether the subject lost the hearing before (from 0 to 1 year), during (from 1 to 12 years) or after linguistic learning (after the age of 12 years), distinguishing a pre-, peri- and postlingual deafness.
From a clinical point of view, the taxonomy is based on the loss of hearing decibels:
If in the first two cases the problems are of a moderate nature, great results can be obtained thanks to the use of hearing aids and good logopaedics. Instead in the last two cases the auditory perception is almost annulled and thus a long and complex logopaedics is needed. Erroneously one tends to consider the “deaf” as these last two categories only.