1.Which is more advisable , to dedicate a few days for studying each section individually or doing mixed sets?

2.How to make the best use of an error log ? I mean that I keep a reference of every question I get wrong (indicating the error type), guess or am not sure of the answer (even if correct). How many times a week do I have to check this log ? and how do I best exploit this data for my improvement?

Thanks in advance