Here are the big ones:
1) Being Arrogant - In a bid to impress co-workers and clients, some interns will talk big and alienate others. The key is in watching specific words you use to convey an idea or your tone of voice. If you say, "Well at Harvard, we did it this way instead of that way", well you immediately get labeled an *sshole and, *ssholes don't get full-time job offers. Other times, you can use the right words but if you say it in a way that is dismissive or somehow conveys you're "better than" the other person, or they are "less than" you, that's a problem too.
I've found some ex-investment bankers and ex-military people have these problems. In banking and the military, there is a more hierarchical culture. In the military, it is okay for a senior officer to insist or even demand a junior officer do something. In banking, it's fine for a more senior person to be more abrupt in dealing with an analyst. In consulting, neither is okay. In consulting, we respectfully ask for what we want (regardless of tenure) and we're polite to everyone. If you're not, you won't get a full-time offer.
Here's an example. If you are a post-MBA intern, and a full-time analyst (who is 5 years younger than you) asks if you would mind ordering dinner for the team because she's swamped and your work load is light, what would you do? In consulting, you'd say, "No problem."
When I was a Business Analyst, my engagement manager offered to order dinner for me when he knew I was busy and he wasn't. I had all the key facts in my head and I was running the deck (getting it produced, double checking it for errors, making final revisions to the details of the presentation) and he was mostly waiting for me to finish. He ordered dinner. No big deal.
2) Using Outward Confidence to Obscure Faulty Logic - Not as extreme as arrogance, but still a problem, is conveying false bravado in lieu of firm logical conclusions.
Everyone knows you're supposed to be confident in consulting. But there's a HUGE difference between being confident that you're correct vs. acting confident hoping nobody will actually check if you are correct.
Consultants have incredibly good BS detectors. If you are logically wrong but acting with great confidence, they will figure it out in less than 5 minutes and call you on it. Do that once, your job offer is gone.
It is better to be confident that you do NOT know the answer, and to explain that you will go find or figure out the answer.
As an example, with my clients I am generally either a) confident I am right and I say so, b) confident that I don't know the answer, and express my lack of certainty with confidence, or c) confident the answer is borderline and there's no obvious right or wrong answer (in which case the decision is more a matter of preference than one based on a data-driven conclusion).
In other words, don't confuse confidence / lack of confidence with right vs. wrong. They are completely independent. Never use one to mask the other.
3) Confusing Opinions with Conclusions & Hypotheses - A conclusion is a statement you can reasonably justify with the facts. A hypothesis is an opinion that hasn't yet been validated by the facts, but is one that you think would be worth validating. An opinion is just an idea.
Never confuse or mislabel these three very different ideas!
Some interns will present an opinion as if it's a conclusion. This is a total job offer killer.
When a BCG intern offers an opinion to a client, and doesn't explicitly frame it as a hypothesis, and the client can disprove it factually, the client doesn't say you screwed up. They say BCG is incompetent.
As you can imagine, partners at BCG (and all the firms) really hate it when this happens. So if you do this just once, and someone important catches you, you're toast. No full-time job offer -- and you probably won't be allowed to talk to a client without "adult supervision" for the rest of your internship.
If you have any opinion, preface it by saying, "Here's an interesting hypothesis..." If it is a conclusion, say, "My conclusion is the client should do X, BECAUSE of X, Y and Z reasons."
NEVER say, "The client should do X" when you're just offering an opinion.
When you don't preface a statement as a hypothesis, it is ASSUMED to be a conclusion.
If you did not intend to state a conclusion, make sure it is crystal clear to the other person that you're stating a hypothesis.
4) Arguing with Feedback - When a manager or partner gives you feedback, it is because you are not doing things the way they want you to do them. 95% of the time, this person is correct and you're incorrect. This is not a dissertation defense. You can't argue your way out of it hoping the constructive feedback won't count.
When you fight the feedback, it counts as 3 times WORSE than accepting the feedback and asking for tips on how to improve.
It is NORMAL for interns and new consultants in their first 3 - 6 months to have numerous problem areas. The firms expect this and build their supervision model around this very common trend. What they are looking for during an internship is whether or not you are receptive to feedback and can improve.
The presumption is if you do not accept the feedback, and instead spend your energy trying to argue your way out of the feedback "counting," then you will never improve. The thought is if you can't even acknowledge there is a problem, there's no way you will take the actions necessary to improve.
5) Making a Computation Error - NEVER EVER make a math mistake. Double and triple check your math. When a client gives you data, NEVER accept it at face value. Always double check the values to ensure the range of values is in the range you expect. If you ask for "sales" data, make sure you know how they define "sales." Is it amount billed? Amount collected? Before taxes? After taxes? Before internal pricing adjustments?
Clients will take a single type of metric and easily have 5 or 6 versions of it. If you don't know the difference, and the senior client catches your misuse of the wrong version of the metric, it is YOUR fault -- not the fault of the client who provided the data to you.
So misunderstanding the data you got from the client also counts as a computational error. In any computation, you need to do two things correctly. You need to do the computation itself correctly (avoid math mistakes), and you need to guarantee the values you are computing are correct and correctly understood.
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