Students need to be fully cooperative and must exert full effort in order to make standardized test meaningful and a good tool for social research. It may happened, as I am going to demonstrate, that the level of effort exerted by the students in these exams varies across and within countries. I will focus in the PISA test and I will try to clean it from these differences in their willingness to answer (WTA). I will measure the role of WTA in standardized tests, and I will show how the use of those tests produces biased results when WTA is not considered.