Demographic data are important for socio-economic planning, political, administrative, research and several other purposes in order to make informed decisions at all levels. The single most important source of such data for a country like Ethiopia is population and housing census. Ethiopia has so far conducted two censuses and preparations are underway for the third one. Given the importance and the immensity of this operation, obtaining quality data is of paramount importance. In this paper, an attempt is made to analyse the quality of data of the last two censuses on the two basic demographic characteristics of a population, age and sex, for the purpose of identifying areas for improvement in the coming third census. Using different demographic techniques, the quality of data from the previous censuses was evaluated and analysis was made on the level of improvement of quality from the first to the second census. The results showed that the quality of data (especially age data) had deteriorated in the second census contrary to expectations. Possible causes are discussed and recommendations made for improving data quality in the planned census.