The situation of a language is always changing, either at the statistical level (number of speakers, transmission rate, etc.) or at the symbolic level (opinion in favour of the language, perceptions, etc.). Sociolinguistic surveys are usefull and necessary in order to know what the situation of a language is in a given society. In the case of Basque we have two main types of survey. One with structured questionnaires and a lot of questions, based on answers of the interviewees : data have their origins in perceptions, self-appraisal, opinions, among others. The second one is based on direct observation and in that way it gathers the real use of the people observed. Nevertheless, it is limited to language use, street use of Basque at the present time. In this paper we examine the methodology of these surveys, their limits and defects, taking some examples concerning Northern Basque Country.