Abstract:

This paper discusses the following main points: 1. Either for combining findings from different countries, or for comparing them it is essential to have truly comparable data. 2. The use of the same methods in different countries does not by itself ensure comparable data because unknown biases may interact differently with different local factors, thus creating differential biases and non-comparable findings. 3. The only way to ensure comparability is to obtain valid data, or data where there are known biases, which can be corrected or allowed for. 4. This known degree of validity can best be attained by using in each country that method which is best and best tested there, first as a valid method, and secondly as a reliable method. 5. The principle of deciding which method to use in each country simply on the merits of the local situation, also leads to considerable cost advantages. 6. The use of different methods in the different countries of a multi-country survey also provides valuable opportunities for checking that one is not simply obtaining consistent wrong answers.