Tim and I disagree on why dosage effects are not observed. He suggests that the data just don’t exist to draw strong conclusions on this point. He might mean that there are very few studies that directly address the length of the intervention (i.e., that make it an experimental factor in the study). That’s why I cited meta-analyses. But I think he’s more focused on the fact that there have been few studies examining interventions that run many sessions.
I’d counter that the length of the intervention ought to have a huge effect. A dosage effect should be easy to observe, even if there were only a few studies because we’re in the early part of the learning curve. Learning curves are negatively accelerated—the initial gains with practice are large, and then gains taper off.
Instead, as Tim notes, we see pretty sizable effects even after brief interventions. That, coupled with the lack of dosage effect, is why I suggested that the mechanism of reading comprehension strategy instruction is not the improvement of the skill of comprehension. Skills don’t show big improvement with brief instruction, and then a lack of continued improvement with continued practice.
Instead, strategy instruction is more like a meta-cognitive technique; it’s a way of organizing and controlling cognition during reading. (I also suggested that models of reading comprehension are more consistent with this interpretation, but Tim didn’t get into that.)
This is why I suggested that spending a lot of time on reading comprehension strategy instruction would be wasteful. I suggested two weeks would be enough. Tim suggests six, noting that my lowball estimate seems rash, given that we both agree that strategy instruction helps. I think that’s a fair point, and I’m happy to bow to Tim’s expertise in the classroom particulars, about which he’s much more knowledgeable.
Tim also notes that, according to a study he’s conducting now of practices in nearly 1,000 classrooms, strategy instruction is not much observed. Large-scaled studies of classroom reading practices are very scarce, so I'm delighted to hear this! I’m doubly delighted to learn that he’s not seeing an overemphasis on strategy instruction. My concerns (opportunity cost and a hit to reading motivation) have been based solely on conversations with teachers and administrators; it will be great to have reliable data regarding frequency.