Math Mistake In Case Interview

pohkid's picture
Rank: Chimp | 4

I made approached the math question the wrong during a case interview, but I corrected myself. Also, I made a minor mistake - the question was about monthly profit and the data was gathered from two months, so I divided by 2 twice by mistake, but corrected myself after the interviewer's reminder. Is that a big no-no? Am I automatically rejected? Thanks for your help!

Math Error in Consulting Interview

To address the OP's original question - it depends. Our users seem to agree that making one math mistake in the case study will not ruin your chances unless you are interviewing with a major hardo. It is important to remember that they are not just looking for your quantitative skills. They are also evaluating your fit, interest, and intelligence.

Dren:

I made two embarrassingly large errors in both my first and final rounds, and still received an offer and extremely positive post-interview feedback for my quantitative skills. For one case, I actually told the interviewer out loud that there were 52 months in a year, and was already half-way done with the math when the interviewer coyly asked me if I knew the difference between a week and a month. I laughed it off.

In a final round interview, the interviewer commented early-on that he was willing to chalk up my estimate to "liberal rounding". I only realized a couple minutes later that he was actually trying to point out a basic math error. Upon realize this, I said excuse me, scratched out my initial number, and promptly did a quick math shortcut to adjust all the following calculations.

While this sentiment was shared by many other users... user @FPM31 explained that a math mistake can be a deal breaker for some:

FPM31:

I would disagree with this; I think a lot of people (Victor Cheng comes to mind) have suggested that if you make a math error and do not catch it during a final round MBB interview, you will not get an offer (this is distinct from asking for a hint, making an error and then realizing it before the interviewer comments, etc.)

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Comments (25)

Mar 6, 2014

i made a math error once during my very first case interview - all i can say is it happens, and don't beat yourself up over it, just take it as a learning experience and make sure next time you're more careful so it doesn't happen again. each case is different, and it depends on your interview experience as a whole, but i personally didn't hear back from them after that. i don't think it was because of my math error, but more so because I simply couldn't connect with the interviewer (I was one of six people in the final super day for 1 offer, and I had 4 separate interviews that day). good luck

Mar 6, 2014

it's probably not a big deal. that is unless you were interviewing for a job as a human calculator, then you're fucked.

Mar 6, 2014

I asked one of my final round MBB interviewers for a hint half way through the case. Still got the offer. Don't stress about it - one little mistake isn't going to ruin your chances.

Mar 6, 2014

I would disagree with this; I think a lot of people (Victor Cheng comes to mind) have suggested that if you make a math error and do not catch it during a final round MBB interview, you will not get an offer (this is distinct from asking for a hint, making an error and then realizing it before the inteviewer comments, etc.)

Mar 7, 2014
FPM31:

I would disagree with this; I think a lot of people (Victor Cheng comes to mind) have suggested that if you make a math error and do not catch it during a final round MBB interview, you will not get an offer (this is distinct from asking for a hint, making an error and then realizing it before the inteviewer comments, etc.)

Eh, yeah, I suppose they're a bit different. I guess what I was trying to say is that assuming the OP performed otherwise flawlessly, I can't imagine a *minor* math mistake ruining his shot at an offer (basically, there are other factors that would probably outweigh this, such as competitiveness of the candidate pool, etc.). That said, I could definitely be wrong...

Mar 7, 2014
FPM31:

I would disagree with this; I think a lot of people (Victor Cheng comes to mind) have suggested that if you make a math error and do not catch it during a final round MBB interview, you will not get an offer (this is distinct from asking for a hint, making an error and then realizing it before the inteviewer comments, etc.)

I made two embarrassingly large errors in both my first and final rounds, and still received an offer and extremely positive post-interview feedback for my quantitative skills. For one case, I actually told the interviewer out loud that there were 52 months in a year, and was already half-way done with the math when the interviewer coyly asked me if I knew the difference between a week and a month. I laughed it off, joked that my brain was doing math in Pluto-time, and went on without issue. Yes, this shit really nagged at me after the interview, but in my opinion you'll have more leniency with math errors if you can clearly delineate exactly what you mean to be doing, and gracefully jump ship if you fuck up.

In a final round interview, the interviewer commented early-on that he was willing to chalk up my estimate to "liberal rounding". I only realized a couple minutes later that he was actually trying to point out a basic math error. Upon realize this, I said excuse me, scratched out my initial number, and promptly did a quick math shortcut to adjust all the following calculations. I got a quick wink but finished the case 10 minutes early and he asked me afterwards what shortcut I used to patch up the math so quickly. In the end, the error may have ironically actually helped my case.

  • Anonymous Monkey
  •  Oct 20, 2015

It is worth noting that a few sections of Victor Cheng's website (market sizing examples) is littered with errors. I actually began writing their support staff an email once with some errors I found in a posting. However, I got halfway through reading for more errors and found too many to be worth my time.

For someone that harps on small mistakes, I would emphasize that making mistakes without pressure and time for review is much worse than a small error incorporated by trying to go too fast under the duress of an interview. Composure means so much. Besides, I've had a friend make and error and come to the wrong conclusion in an interview with a parter at BCG and get an offer.

One more thing. The Starbucks down the street here in Cambridge gets 70 applications for one barista opening, that doesn't mean it's harder to get a job at Starbucks than get into Harvard. I can't help but sense a little vanity show through when I hear him and others make those comparisons with how elite being a part of a top firm is with their 4 % acceptance rate. Getting through the process and getting hired is hard and takes hours but please.

Make sure you are so well prepared that you can catch yourself or correct a small mistake you may make with flourish and poise.

Mar 7, 2014

I just finished a very successful internship recruiting season and advanced/was offered after small math errors. They are not an automatic killer, but they certainly don't help you. I found if I had written out (in words) the formula (e.g. # of days x trains/day x cost/train = total cost) and then botched the arithmetic, I got the benefit of the doubt. If you were to say "well, if I take 7 and multiply by 4 and by 500...," you're less likely to get the pass, because it's not clear what you're doing.

Oct 20, 2015

Also should add that I am in my 4th round of interviews and it is myself and one other candidate

Oct 20, 2015

Sit tight. DO NOT inform them of the error. It may go unnoticed. And alerting them to it now doesn't do anything for your candidacy. You already made the mistake, and from where they're standing you didn't know how to deal with the calc and went and looked it up and now you're following up. This wasn't a take home test. You're better off letting it ride and maybe it goes unnoticed or maybe they notice and think its a careless mistake and let it slide.

While its only natural to think that your case study model needs to be completely flawless and 100% error-free, practically speaking no one is going to do a cell-by-Cell audit of every single line of the your model. What's more is that there is a certain number of assumptions you can make on your own that would slightly impact CFs and returns but wouldn't be wrong. For these reasons, the model test checking process is typically a few specific datapoints... like a check list of say: FCF at the end of year 3, cash balance at the end of year 4, equity check at entry, IRR at exit, balancing balance sheet and maybe or two other random mish-mosh of datapoints to compare vs the answer key.

As long as the numbers are in the ballpark, its not going to be a big deal. If your IRR was 23.7% vs. the answer key IRR of 23.9%, I don't think its going to be enough to raise any eyebrows. If its was 23.7% vs. 27.5%, thats obviously a meaningful difference and they may poke around to figure out why your math is different.

But they'll generally take this in the context of your broader candidacy. I.e., if there are 3 other candidates with flawless models but they really really liked you best, and it was only a minor error that doesn't suggest you have a fundamental deficiency in your modeling/accounting/financial analysis skills, they'll be more forgiving.

    • 1
Mar 7, 2014

Small math errors are not a big deal at all. The firms care far more that you demonstrate calm, composure, and professionalism when you screw up.

Mar 7, 2014

Well, according to consulting guru Victor Cheng, math errors are a big deal. Who really knows?

Mar 7, 2014
zoomi:

Well, according to consulting guru Victor Cheng, math errors are a big deal. Who really knows?

I made small math errors and it made no difference, so YMMV.

Mar 7, 2014

If they don't like you? Your errors will only reinforce the decision that they probably already made. If they do like you? They'll justify your errors as inconsequential.

Either way, it's out of your hands right now, and likely won't be a deciding factor in either case.

Mar 10, 2014

very true!

fight for MBB

Mar 11, 2014

Shockingly, someone who sells case interview training preaches that mistakes can kill you. Of course he does - that's his business. I'm not even criticizing him (I think his product is solid), but it's not really that difficult to realize that someone that makes his living off selling case interview secrets will possibly go a bit overboard emphasizing the importance of the little things.

    • 1
Mar 18, 2014

Victor Cheng's stuff is quite good -- it certainly helped me. But don't read too much into what he says. I had a somewhat similar experience as Dren. During a final round interview, I messed up a very easy math problem, and spent what seemed like an eternity panicking over it. I ended up not solving it, but apparently it didn't matter to them :-)

Mar 21, 2014

If you get dinged it won't be because of a little math error.

Mar 23, 2014

Depends on the interviewer. I personally will point out the first math mistake and not hold it against the candidate, but after that will begin to dock points.

Mar 23, 2014

I made a math error in a phone case interview. Overestimated time to make ROI on a new product to automate service calls/improve customer service. Cost consisted of initial investment+yearly maintenance fees+cost of addition of new customers yearly, savings came from job cuts, vehicle gas savings, insurance savings, and other misc. costs. Calculated ROI to be 15 years and suggested against it based on other factors. During the behavioral (I did case first and then behavioral with the same guy), I double check my math and realized ROI was around 11.5 years. Brought it up to my interviewer and said I still suggest against it but not as easy of a decision, conversation below:

Interviewer: (Laughs) So you are changing your advice?
Me: No, just making sure I acknowledge my mistake and give the client the most accurate answer, being accurate beats being too prideful to admit mistakes.
Interviewer: That's a d*mn good point, well done.

BOOM Interview saved

Mar 27, 2014

I like your statement of "being accurate beats being too prideful to admit mistakes." Well played.

Mar 26, 2014
pohkid:

Am I automatically rejected?

No way to answer that question seriously. You'll get the decision soon enough.

pohkid:

Thanks for your help!

One advice: get over it. You will make more mistakes in your life, learn to deal with it.

Mar 27, 2014
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Mar 29, 2014
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I previously worked for McKinsey in London and have started a blog about consulting and how to get into it at www.theconsultingcoach.com