John Hattie's "Visible Learning" research aims to identify "what works best in education" by synthesizing a massive amount of existing research, primarily through meta-analyses. His work involves pooling effect sizes from over 800 meta-analyses, encompassing over 50,000 studies and millions of individuals. This colossal synthesis, which quantifies the impact of various educational interventions on student achievement, has earned his book "Visible Learning" the reputation of a "Bible" or "Holy Grail" in education. Its appeal lies in its apparent scientific rigor and the promise of concrete data to guide educational practices, offering what seems to be a clear ranking of factors influencing student learning.