How many piano tuners are there in London? (Friday, Jun. 10, 2005)

Thanks to an ex-colleague, this question is one that I often ask people who are applying for a developer/analyst role at the company I work for. The aim of the question is to see how people analyse what is a seemingly impossible question, to come up with a reasoned solution. For example, you can probably hazard a guess at how many people live in London, and based on the people you know, have a rough idea of what percentage of those people own a piano. It's then a case of trying to estimate how often a piano is tuned, and how long it takes to tune. Hey presto, you have enough information to work out *roughly* how many piano tuners there are.

Usually potential candidates have a pretty good stab at this, some give exceptional answers, some simply answer don't know, so I try and lead them to the right answer. Todays interview was however truly, truly, painful - the candidates name has been changed to protect his identity:-

Me: So, how many piano tuners are there in London?
Bungle: 50 thousand
Me: OK...so how did you get that answer?
Bungle: It's a guess
Me: OK, so explain to me how you might work out an answer?
Bungle: Er...still think it's 50 thousand
Me: Right - let's start working through this...how many people do you think live in London?
Bungle: Er....5 million?
Me: Good - so what percentage of those people do you think might own a piano?
Bungle: Er....100 thousand?
Me: Riiiight, and how often would you expect them to tune their piano? Do you see where I'm going with this?
Bungle: Ahhhhh, OK, yes I see what you mean
Me: OK, so now you know what I'm after, I'll ask the question again - how many piano tuners are there in London?
Bungle: 50 thousand
Me: .......so lets work through it again....we've decided that 100 thousand people own a piano - how often do you think they would need it tuned?
Bungle: er, not very often....once a week?
Me: OKaaaaay, so based on that, how many piano tuners are there in London?
Bungle: er......50 thousand?
Me: [Visualising beating Bungle around the head with a leg of the table] But how do you get to that number?
Bungle: Aaahhh, sorry I see...you divide 100 thousand by 2
Me: [Perplexed look - have replaced table leg with chainsaw] And why would you do that?
Bungle: Simple really, 100 thousand divided by 2 is 50 thousand
Me: Good...lets move on to the next question, How long do you spend planning tasks as opposed to doing them?
Bungle: er...2 hours, on a Monday
Me: Well thanks for coming in, we'll be in touch.

The sad thing is we'll probably offer him a job...

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