Problems for the comparison with public opinion surveys.

To compare different data coming from public opinion surveys is always a problem for the social researcher. From a longitudinal perspective, the comparison of data coming from distant points in the time is difficult when the data are not coming from panel. If we compare data whose geographic base is different, we can also find some limitations. In this work we consider different methodological aspects to make comparisons las much from a general point of view like individual). To compare in the time or with different units taking public opinion surveys like a tool for collecting data. Particularly, we introduce the example and the limitations to establish comparable temporary series for the Valencian Community; and the methodological and technical problems that the comparison of data for the Basque Country and Spain presents, with databases and surveys from CIS (Spain).