How would you build a business case for replacing an old core system, including costs, benefits and risks?

What the interviewer is looking for

• This question tests solution evaluation. Builds business cases with costs, benefits and risks, and checks after go-live whether benefits were actually delivered.
• Strong answers: Covers options including doing nothing, total costs, quantified benefits, risks and assumptions, and a phased approach with decision points.
• Likely follow-up: Which numbers in a case like that are most often wrong?

Another way it may be asked: The company is thinking about replacing a twenty year old system. How would you help leaders decide whether it is worth it?

A structure to follow: Frame, structure, analyse, recommend

1. Step 1

   Frame

   Restate the goal and ask what success is and what the constraints are.

2. Step 2

   Structure

   Split the problem into a few separate parts you can check one by one, and say what you will look at first and why.

3. Step 3

   Analyse

   Reason with the information you have, state your assumptions out loud, and say what data would change your mind.

4. Step 4

   Recommend

   Give a clear recommendation, the main risk, and the first step you would take.

This is guidance on shape, not a script. Use your own real experience and facts; never invent details to fill a gap. See the STAR guide and answer frameworks.

Common slips

• Listing ideas without choosing between them.
• Hiding assumptions instead of stating them.
• Finishing without a recommendation.

How to practise it

Answer in four spoken parts, one per step, then check you ended on a recommendation.

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