Case interviews: a preparation plan that starts from zero

A structured way into case interviews: the four case types, how to build a structure out loud, and the arithmetic drill that removes most of the pressure.

The short answer: A case interview asks you to reason through a business problem out loud, and it is scored on structure, quantitative comfort, insight and communication — not on reaching a predetermined answer. There is no answer; the interviewer is watching how you decompose a problem you have not seen before. Preparation from zero takes roughly four to six weeks and has three components: learning to build a structure you state out loud before analysing, drilling mental arithmetic until it stops consuming attention, and practising live with another person rather than reading cases silently. The last one is where almost all the improvement comes from, and it is the one people skip.

Key points

State your structure out loud before you analyse anything — an unstated structure cannot be scored or corrected.
Mental arithmetic under observation is a separate trainable skill from arithmetic, and it is worth drilling on its own.
Practise live with a partner; silent reading of case books produces very little transfer to the room.
Ask clarifying questions before structuring, then commit — endless clarification reads as an inability to decide.

What is actually being scored

The four criteria, and what each one looks like
Criterion	Strong evidence
Structure	States a clear, case-specific breakdown before analysing, and follows it
Quantitative comfort	Sets up the calculation correctly, works out loud, sanity-checks the result
Insight	Draws a conclusion from a number rather than just reporting it
Communication	Signposts, is concise, and gives a recommendation the client could act on

Notice that "reached the right answer" is not a row. Cases rarely have one, and two candidates with different conclusions can both score well if the reasoning holds. What cannot score is analysis that arrives without a stated structure, because the interviewer has nothing to mark it against.

Structuring out loud

The habit that most improves a case performance is announcing the plan before executing it. Take thirty to sixty seconds of silence, write down three or four branches, then say them: "I would like to look at three things — the revenue side, the cost side, and whether the market itself has changed. Shall I start with revenue?"

That sentence does three jobs at once. It shows the structure, which is the highest-weighted criterion. It gives the interviewer a chance to redirect you before you spend ten minutes in the wrong branch. And it makes the rest of the case legible, because every subsequent statement has a place in a stated plan.

A structure built for the case rather than pulled from a book

The prompt: A regional bank's cost-to-income ratio has worsened over three years. Why?
Branch 1: Income: has revenue fallen, and in which product lines?
Branch 2: Cost: which cost lines have grown, and are they fixed or variable?
Branch 3: Mix: has the business shifted toward lower-margin products?
The question that follows: "Do we know whether income has fallen in absolute terms, or only relative to costs?"

Three branches, all specific to the ratio in question, stated in about twenty seconds. This is what a structure is; it is not a framework, and it is better than one because it fits.

A four-week plan from zero

Week 1 — mechanics. Read enough to understand the format and the four criteria. Start ten minutes of daily arithmetic drills: percentages, ratios, growth rates, order-of-magnitude estimates, all mental.
Week 2 — structuring only. Take ten case prompts and build a structure for each without solving anything. Say each one out loud. Fifteen minutes per prompt.
Week 3 — full cases with a partner. Five to eight complete cases, alternating who gives and who receives. Giving is as instructive as receiving because you see what vagueness sounds like.
Week 4 — pressure and polish. Ten more cases, timed, with the partner deliberately interrupting and challenging. Add the market-sizing and estimation formats if they have not appeared yet.

Find the partner first, because everything else is easy and this is the step that fails. University consulting societies, alumni networks and peers applying to the same firms all work, and so does the informational interview approach with someone a year ahead of you.

Frequently asked questions

How long does case preparation take?

Four to six weeks of consistent work from a standing start, meaning roughly twenty to thirty live cases with a partner plus arithmetic drills. Beyond about thirty-five cases returns fall off sharply and answers start sounding rehearsed, which is itself penalised. Little and often beats an intensive week.

Do I need to memorise frameworks?

No, and visibly applying a memorised framework is a recognisable negative signal — interviewers see it several times a day. Learn the underlying logic of a few, then build a structure specific to the case in front of you. A structure with three genuinely relevant branches beats a textbook framework with two irrelevant ones every time.

What if my arithmetic is wrong?

Say so, correct it, and continue. Interviewers expect errors under time pressure and are watching how you handle one. What is penalised is not noticing — presenting a figure that is off by an order of magnitude without pausing suggests you would do the same in front of a client. A visible sanity check protects you from the version that actually costs marks.