You are correct regarging the fact that your notes shoud be clear and neat, so that interviewers can follow your thought process easily and that issue trees are key to having a sound, structured approach to solve a case.
A good way to think of issue trees is to consider them as hypothesis testing structures. After all, the main point of using issue trees (aside from organizing your ideas) is to evaluate whether the hypothesis at hand is true or false. Issue tree help in such evaluation by breaking down the problem into its elementary / fundamental parts. Please refer to the example below:
Situation: Client is losing profitability
Hypothesis: Client can improve its profitability (which composes of Revenues - fundamental part #1 - and Costs - fundamental part #2)
- Condition #1: Client can increase its protability by increasing its revenues
- Condition #2: Cient can increase its profitability by decreasing its costs
Having that conceptual understanding of issue trees in mind, it's good to have a simple roadmap to building good issue trees:
- You have to have a perfect, clear understanding of the problem being asked before you start
- With a clear understanding of the problem, you have to think of a reasonable hypothesis to test
- With a reasonable hypothesis at hand, try thinking of what conditions must be met in order for it to be true; Also, try thinking of what sub-conditions must be met in order for each of your main conditions to be true
- Guarantee that the conditions to test your hypothesis are MECE when you put them together
This little tweak in the understanding what issue trees are for and that simple roadmap typically helps candidates improve their issue trees builing abilities. I hope it helps yours.