In 2007, Hattie and Timperley discovered from their meta-analysis of almost 8,000 studies that feedback is nearly seven times as effective in improving student learning as reducing class size. They found that feedback is "the most powerful single modification that enhances achievement."
Feedback is effective when it consists of information about progress, and/or about how to proceed.
Hattie, J., & Timperley, H., (2007). Review of Educational Research. The Power of Feedback, 89. doi: 10.3102/003465430298487