Research study that found an explicit intervention in phonemic awareness, decoding, spelling, fluency, and comprehension demonstrated stronger impacts on student reading than did a Guided Reading intervention. On almost every measure in the study, the explicit instruction group demonstrated accelerated progress beyond that of the Guided Reading group. Here is the full reference:
Denton, C. A., Fletcher, J. M., Taylor, W. P., Barth, A. E., & Vaughn, S. (2014). An experimental evaluation of Guided Reading and explicit interventions for primary-grade students at-risk for reading difficulties. Journal of research on educational effectiveness, 7, 268-293.