I've been following your site since I received an interview invitation from BCG. It takes place soon, but a partner emailed me to offer me a practice case study interview if I was interested (not by him but by an associate).
I of course accepted it. My question is whether this is truly practice or a sneaky way of getting a another screening of me.
In other words -- if I bomb it, do I realistically lose the job before I even get my real interview?
Hmm... I think it is best to assume this is a genuine offer.
The firms who offer practice interviews often do it as a standard case interview practice for either everyone, or everyone with a similar background (often non-business background types).
The practice interviews at other firms really are practice interviews. If you do poorly, it does not count against you. If you do well, the practice person might mention it to your actual interviewers -- but mostly likely not, as people are pretty busy.
If you have a particular problem area that the practice person notices, he/she will definitely tell you that is your weak spot, and might tell your actual interviewer to really test that particular area in depth.
Best case -- it's pure practice and nothing else.
Worst case -- the actual interview will focus more on your weaknesses -- but you will have the advantage of knowing what those are, and have some time to work on them.
To understand the rationale behind this -- the firms generally want to see how good you are at cases. They want to see if your best is good enough vs. trying to trick you into see you at your worst.
The reality is case interviews are not typical interviews. As much as a candidate does not like doing poorly on a case, keep in mind, interviewers do not like it either!
It is painful to watch someone struggle in a case.
I've also noticed a trend of more firms offering practice case interviews with an actual practice interviewer. This has got to be a pretty expensive investment on their part to do this.
It begs the question, "Why do they do this?"
My best guess is they analyzed their recruiting track record and noticed that a significant enough portion of early round rejections were errors... meaning the candidate was actually good, should have been hired, but did not make it through the early stage interviews.
My next guess is that most of these rejections came from candidates who had format unfamiliarity, rather than the lack of analytical skills.
For example, my roommate in college was much smarter than me. He had a science math background and had perfect grades (or close to it). I had more of B+ / A- grades in less technical coursework.
But, he got rejected McKinsey Round 1. The interviewer gave him an estimation question, something like "Estimate the number of shoes sold in the U.S. last year."
His answer was, "What... you mean like you want an actual number?"
Then 28 minutes of dead silence.
It was the first time he had ever heard of an estimation question. He was smart enough to do the analytics, but was completely unfamiliar with the process.
My guess is that many of the early round rejections at some point were due to reasons similar that this. So various firms started to offer practice interviews to have candidates be more prepared.
Also, partners in particular hate interviewing candidates that totally bomb a case. They prefer trying to assess the difference between someone who is good vs. excellent.
Many feel it is a waste of their time to interview someone who is either totally clueless about the case process (and should have been made familiar with the process before the partner interviews the candidate) or someone whose performance was so poor, they should have been weeded out earlier in the process.
Hope this helps shed some insight onto this topic.