I've done chapter 6 of D2 only, the rest looks so boring...
S3 is a bit long but it's interesting. 5 chapters. First chapter's dead easy (combining 2 normal distributions, just some adding and multiplying), second chapter is boring (sampling methods, you got to memorise them), third chapter is the most important (it's the only real 'proofy' part of S3). 4th chapter is goodness of fit tests, for example, proving that a set of data can be modelled by a certain type of distribution. Final chapter's really easy, just spearman's rank and revisiting pmcc from S1.
S4 is really short, only 3 chapters but tricky to remember the formulae for t-distribution and Fisher-distributions, so many different kinds of hypothesis testing. Basically S4 is all about hypothesis tests, but they really shell out everything (hypothesis test for variance, standard deviation, one sample procedures, two sample procedures, etc). S3 is more interesting.
I really need to learn D2 (and D1). But got chem and physics revision to do.
Doing all of them to free up time in Year 13. Hoping applying to uni with 3 A-levels will help me a bit. What about you?