So, first it would be good to know what business we're talking about! That said, in general, you want to take the following approach.
- Determine whether this is industry-wide or company specific
- Break down revenues into logical segments (by product, geography, etc.).
- Look again at overall trends across each segment and split by price and volume
- Aim to determine across the biggest trends/segments, what is actually happening (i.e. price war, competition, x customer group no longer buying, etc. etc.)
General Case/Framework Advice
How do I use frameworks in a case?
If there's anything to remember in this process, is that cases don't exist just because. They have come about because of a real need to simulate the world you will be in when you are hopefully hired. As such, remember that they are a simplified version of what we do, and they test you in those areas.
As such, remember that a framework is a guide, not a mandate. In the real-world, we do not go into a client and say "right, we have a framework that says we need to look at x, y, and z and that's exactly what we're going to do". Rather, we come in with a view, a hypothesis, a plan of attack. The moment this view is created, it's wrong! Same with your framework. The point is that it gives us and you a starting point. We can say "right, part 1 of framework is around this. Let's dig around and see if it helps us get to the answer". If it does, great, we go further (but specific elements of it will certainly be wrong). If it doesn't, we move on.
So, in summary, learn your frameworks, use the ones you like, add/remove to them if the specific case calls for it, and always be prepared to be wrong. Focus rather on having a view, refering back to the initial view to see what is still there and where you need to dive into next to solve the problem.