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Hi Mark,
Sorry for the delay in responding! This is a good question, and a very clear, concrete one, but I don’t know if there’s an equally concrete answer or set of “best practices” that addresses this.
Mostly because I think it’s all kind of a moving target. I think a helpful way of conceptualizing interleaved practice is to think of it as one way to increase the difficulty of a task. So in the early stages of learning a new passage/skill, you want to reduce the difficulty, so you can stay in your “zone of proximal development” as it were. Or that slightly uncomfortable place just outside of what you can do currently, so it’s not too overwhelming, but you’re still pushing yourself slightly.
And as you get more skilled and comfortable with the passage, you can start adding in more interference, or additional challenges, whether that be expanding your dynamic range, or phrasing something in a more extreme way, or using a different type of vibrato, aiming for a more specific type of sound, etc. And interleaving your practice can add more of a challenge to this too, as the interference from other passages/pieces can make it harder to get things right the first time.
Different passages/skills have different levels of difficulty, and different trajectories of change/improvement, and we each have different rates of improvement too on different types of skills/tasks. So it’s a lot fuzzier and messier than I think we’d like it to be, but I suspect the answer is that we have to monitor each passage/skill that we’re working on, and be responsive to what that particular passage needs at this moment in time, relative to where it needs to be, for whatever performance we have coming up.
Hope that helps to answer your question!
Noa
