In the good old days of PW we had discussions about interleaved practice. If you want to go back and read there, there is a thread called interleaved practice, but there is also quite some discussion about it in Holly's practice diary.
Anyway, Molly Gebrian discusses interleaved practice (she calls it random practice) in the second video of another series of videos about how to practise. It would be good to watch the first of the series as well, so you understand everything she says in the second one.
From the video:
Blocked practice is to practice each thing you practise for a block of 30 minutes, and then move on to another thing for a new block of 30 minutes.
Random practice, also called interleaved practice, is to practice each thing for three times 10 minutes at a random order.
This sounds chaotic and unfocused but the science says that random practice is better. One of the reasons is that in blocked practice, you get many chances before you play something perfectly, whereas on stage, you need to get it right at your first try.
They did also a study with pianists who had to learn new pieces. During the practising, it seemed that those who did block practice were doing better, but during the performance two days later, those who did random practice performed much better.
But, the pianists themselves thought that blocked practice was much better. This phenomenon is so common that it has been given a name: the illusion of mastery. This is because when you practise, after many repetitions you can play it perfectly. But that is very different to playing the piece perfectly on your first try.
After the discussions on interleaved practice at PW, I also started practising like that. But slowly I reverted to blocked practice. I think one of the main reasons was that blocked practice feels better than interleaved practice. But now I understand that this is a trap. 😬
In one study though, baseball players either did blocked practice, random practice or increasingly random pattern of practice: first blocked practice (AAA BBB CCC), then serial practice (ABC ABC ABC) followed by random practice (ACB CBA BCA). This increasingly random schedule was the best of all.
This is very interesting, because it says that blocked practice can have its place in the beginning, when you are learning something new. So start with blocked practice. As you get closer to a performance, transition to more random practice.
In the first video of this series, she mentions that when you fix a mistake, you'll need to play it ten times correctly in a row. This is also blocked practice. So blocked practice has its usefulness. However, if you only do blocked practice, your brain does not practise what it needs to do when you perform.