Why is the research focused on one dimension of schooling i.e., student achievement?

The Visible Learning research has achievement as its focus. The research covers the typical range of achievement expected in schooling contexts. It includes many outcomes related to numeracy and literacy, as well as social studies, science etc. It includes many formats including standardized tests, state tests, teacher made, researcher made tests, etc. Of course, there are many other outcomes of schooling such as retaining students’ interest in learning, affective, health outcomes as well as respect for self and respect for others. John has kept to one outcome as it is so important.