Math Error in McKinsey Round 2 first interview

Question

Hi, so I just concluded my McKinsey final round interview (1 of 2) - the interview was structured normally, a PEI section followed by a case interview. I felt I performed well on the PEI and was able to answer all questions the interviewer had about my experience in a structured manner.

The case itself was a public sector case, greenhouse gas emmissions for a country, particularly focusing on the power generation industry. My overall approach was highly customized, well structured and properly communicated (numbererd buckets, clearly speaking, and making relevant statements with regard to the country in question) - the interviewer said she was satisfied with the structure.

Following this, I got an exhibit with a number of data points, here I was quickly able to share level one insights in a structured way as well as calculate level two insights and share those as well. In addition, I connected back to my orignal hypothesis and structure confirming certain parts of it for a deeper analysis.

Here comes the problem, following this I had to do what I would call a simple math calculation - calculate the percentage change in emmissions in the previous period, double that, and apply it to the current period to see what the end result would be of the proposed strategy (told in the prompt) - I laid out my approach, broke it down into steps and confirmed the approach with the interviewer (in laying out and confirming I explicitly said I would double). I talked through all the math and did it correctly BUT missed out on the doubling of the percentage change.

This ofcourse threw off my final answer.

My follow up 'so what' was good in response to my calculation (which was off of course) but here's the thing, the interviewer DID NOT point out the concern or anything. All she said was wonderful thank you and we moved on in the interview to the closing conversation.

An important point to note, the final conclusion was correct just off in the gap i.e. country was NOT meeting target but by how much I had calculated wrong so, my follow up 'so what' was correct as well as the final reccomendation.

Overall the interview vibe was good, smiles on both sides and good communication overall so I want to understand:

a. Why did she not hint towards the error b. How badly am I screwed here

Please do let me know, thanks.

Answer 1

Hi there,

Great question with all the detail!

I can confidently say that the reason she didn't point out the error is that overall you did very well and a little mistake here or there doesn't change the overall impression. Everybody makes mistakes, especially mistakes of ommission, so why correct you? If everything else is as you describe, you did well enough.

If you had other problems, she might correct you and see if you are able to fix your mistake.

It's not like studying with a junior case practice partner. You don't have to get everything on the page to do well.

Bottom line, you're not screwed!

Best of luck,

Allen

Answer 2

a. This is what professionalism means - it is up to her to decide whether to let you know or not - she certainly noticed it.

b. No one will be able to tell you for sure - here are my thoughts: When I interview people, the most important for me is the approach and that the candidates walks me through the approach in a transparent way and gives me a chance to react. The actual math portion is not too important to me because they already proved to me that they know how to solve the problem, just messed up because of anxiety or so. Seems you think your approach was correct, your so what, and the final recommendation - so should be good. However, of course, as always, you never know and that interviewer puts a lot of weight on your little mistake there. Problem is, they dont know if you are just nervous or you do not have a systematic eye for details and your work is sloppy. Cannot say what is applicable in your case without interviewing you. However, there are probably as many opinions as coaches on PL.

I think you will be fine.

Answer 3

Hi there,

First, it sounds like you did well, but we really won't know for sure until we get the final answer. Here are my thoughts.

a. Why did she not hint towards the error

It did not matter at that point. I.e. she had already made up her mind about you as a candidate (for better or worse) - she didn't want to delay the case.

b. How badly am I screwed here

You are not screwed. We can't say you got in, but this one incident is, in of itself, no final indication of failure.

Answer 4

Hello!

In any ways congrats, it´s done!

By your very detailed post I would say you did well, but it´s impossible to know how "bad" the mistake you describe without having been there.

My suggestion would be to try to forget about it and just wait, there is nothing you can do in any case.

Hope it helps!

Cheers,

Clara

Answer 5

Hi there,

There may be multiple reasons:

The interviewer didn't notice
The interviewer did notice, considered the mistake marginal taking into account the overall performance
The interviewer did notice, already considered not to let you pass so considered irrelevant to point this out

It seems you did well overall so 3 may be unlikely, but you never know.

Hope this helps,

Francesco

Answer 6

Hey there,

Already some good answers here so I just want to add one point!

One mistake like this is not the end in a McKinsey interview, provided

it is a one-off and in all other cases your math was stellar (they will check with their fellow interviewers)
your performance in structuring and chart interpretation questions was good or excellent

Fingers crossed!

Cheers,

Florian