An established typology which attempts to analyse the linguistic evolution of individuals, that is, what their first language is and what their current level of Basque is. In order to do this, a cross of variables of linguistic competence and first language is carried out, and the result of this cross is the BILA or linguistic mobility index.

This index is composed of seven categories:

. bilinguals, according to their first language, give rise to three groups (native Basque speakers, native bilinguals and new Basque speakers)
. passive bilinguals give rise to two groups (new partial Basque speakers and partially hispanicized) and
. monolingual Spanish speakers to another two groups (totally hispanicized and native Spanish speakers).