When consulting firms offer you an internship, they are more or less hiring you for a full time job, but with a 90-day trial period.
So on Day 1 of your internship, for all practical purposes you essentially start with a full time job offer in hand. The goal of the internship is to not lose the offer.
Now in practice, it is nearly impossible to totally impress your firm in the first 90 days of working there. You're just too new and too inexperienced.
You can't impress your firm because the firm will not give you enough responsibility to really "wow" them.
For example, at McKinsey in my second year, I was rated in the top 10% of all consultants in the world for my start class.
However, when I look back to my first 90 days at McKinsey, my performance was incredibly unremarkable. I frankly did not do anything that impressed anyone.
The kind of work I did was very contained in those first few weeks. I didn't have any major insights on anything, in large part because I didn't work on anything substantial enough to gain access to data that would be sufficient to generate an insight.
What I DID do well in the first 90 days was AVOID making a catastrophic mistake - a mistake that did or could have jeopardized McKinsey's reputation.
Unlike the case interview, where you really want to "wow" the case interviewer, in an internship you just want to avoid making major mistakes and you will get a full time job offer.
So what are some examples of the reputation damaging mistakes?
Let me give you two examples (and provide an explanation, as these mistake don't seem like they would be that big of a deal, but to MBB they are a huge deal).
1) Making a computation error
2) Making a conclusion not supported by data
If you work for BCG and you present to a client that 2 + 2 = 6, it is a HUGE FREAKING problem for BCG.
The senior client thinks to herself, "I am paying $1 million per month for this? You got to be kidding."
The partner is thinking, "Oh crap, I can't believe my intern just did that that. Ugh... I might lose this client. Ugh... I'm up for making Managing Director in 4 months and this stupid, stupid intern might cost me my directorship."
Of course the partner will never say this out loud to anyone, but it IS what she is thinking.
When you cause a partner this much of a headache, they don't extend you a full time job offer.
Similarly, if you try to impress a client by expressing some opinion as to what the client should do in a particular situation but you don't back it up with data, that is a HUGE problem for BCG too.
In business school (or other schools), you can often get away with making up some bullsh*t comment to score some class participation points with the professor. In consulting, you absolutely, positively, can not do this. You can not confidently express an opinion not backed up with data.
Your conversational opinion, designed to make you sound like you know what you're talking about, will NOT be seen as your opinion. It will be seen as a recommendation of The Boston Consulting Group -- with the full backing of the reputation of the firm.
You MIGHT be able to fool the client, especially if they don't challenge you.
But some clients will challenge you.
"So you think we should enter XYZ customer segment. Why?"
Let's say you think quickly on your feet and give some answer, any answer, to avoid looking like a fool.
Then the client says, "and on what data do you base that conclusion?"
This is the point where your manager and the partner start to sink into their chairs, and say "Uh oh... we are in trouble."
A good manager or partner will immediately cut you off and redirect the conversation. They will do so to avoid even more damage to the firm.
There is an expression in the U.S., "It is better to be thought a fool, than to open your mouth and prove it."
A partner or manager that cuts you off is saving you from proving you are a fool.
Regardless of whether or not you can fool the client into thinking you know what you are talking about, you will NEVER fool the partner or your manager.
They will know if you are bullsh*ting the client or not. They will know because if you are wrong (which includes saying the right thing but without any proof), they are the ones who will end up contradicting you later in the engagement.
The problem is it won't be seen as your partner or manager contradicting you.
It will appear, or risk appearing, to the client that BCG is contradicting itself.
The fear the partner has is that the client will be thinking, "Hmm... We should have hired Bain or McKinsey instead."
Incidentally this is WHY the firms are so strict in their performance standards for the case interview. It's because that stuff actually matters on a day-to-day basis in consulting.
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Succeeding in a Consulting Internship