Practice by question type is one of the fastest ways to fix a specific weakness.
If you keep getting Venn diagram questions wrong, doing a mixed DM set may only give you one or two chances to practise them.
A subtype set lets you repeat the same skill until the method becomes familiar.
This is especially useful early in preparation or after a mock reveals a weakness.
The danger is staying in subtype practice forever.
Once you become accurate, switch to mixed sets so you learn to recognise the question type without being told what is coming.
A good progression is:
learn subtype → practise subtype → time subtype → mix it with other questions
That moves the skill from isolated practice into real exam performance.
