From "Elements of Grading" by Doug Reeves

"Certainly, Hattie is not the first scholar to recognize the importance of feedback on student achievement. His findings are completely consistent with Robert Marzano's conclusions that accurate, specific, and timely feedback is linked to student learning. Thanks to Hattie's research, however, we can now be more precise than ever about how important it is. We can say, based on the preponderance of the evidence from multiple studies and many cultural settings, that feedback is not only more important than most other instructional interventions, it is also more important than socioeconomic status, drug use, nutrition, exercise, anxiety, family structure, and a host of other factors that many people have claimed are overwhelming. Indeed, when it comes to evaluating the relative impact of what teachers and educational leaders do, the combined use of formative evaluation and feedback is the most powerful combination that we have."