Essentially, you deal with guesstimates in the following manner. Imagine you are asked a guesstimate in an interview – “How many people in Mumbai wear red on a Monday?”
Your first task after you hear the question should not be to start solving immediately, as there are several things that need clarity. So, pause for a few seconds, gather your doubts and start clearing them one by one.
In the example provided above, let’s assume a candidate Malhar was asked this question. The conversation could have been something like this –
Interviewer - Hi Malhar. So, the guesstimate for you today is quite interesting. Guesstimate the number of people who wear the colour red on a Monday in Mumbai.
Malhar - Sure Sir! Thank you for the question. Would it be alright if I take a few moments to gather some clarifications I might need on the question statement?
Interviewer - By all means. Take your time.
After 30 seconds
Malhar - Thank You. So, after a bit of deliberation, I have a few doubts I would like to clarify before I begin solving my guesstimate. To begin with, what is the extent of red? Should their clothing be red? Is a simple red coloured tie counted as red enough or does it have to be a major part of their dress, for example, a shirt or a pair of trousers or blouse or dress, and any other type of clothing?
Interviewer - That is a good question, Malhar. Let’s go ahead with the latter approach of the two.
Malhar - Alright. Second, are colours like maroon or any other closely resembling colour, which fall in the hue of red, considered red or is it purely red?
Interviewer - For the sake of simplicity, let’s assume colours that resemble red can be factored in.
Malhar - Perfect. Finally, factoring in that people change their clothes in the evening generally, does it need to be at least one red clothing in the entire day or a minimum of one red clothing throughout the day?
Interviewer - Let’s go with at least one red item of clothing for the entire day.
See, that is how easy it really is. All you need to do is not rush in to start solving. Pause, think and come back with legitimate doubts.
Next up, you align your approach to solve the guesstimate. At this step, however, things become a bit different for the different types of guesstimates. Let’s discuss them one by one.
Geometry or Shape Related
If you are faced with this type of guesstimate, the best approach is to simplify it using the formula of area and volume of a simple figure that the structure might resemble. For instance, a question on an aeroplane can be solved using the formula of a cylinder, a car with the formula of a cuboid and so on for other complex structures.
Keep in mind that even if you do not remember the formula, it is fine to ask the interviewer or simply let them know that since you do not know the exact formula, you would apply a certain workaround OR assume a formula based on which you can recollect, though it might turn out to be wrong. The interviewer is not interested in knowing how well you remember the volume of a pyramid; instead, he wants to know how well you manage to solve it despite not knowing it. The idea is to tackle problems at every step.
Market Sizing
Such guesstimates can be solved from two different sides – the demand side or the supply side. You can choose whichever you find easier. For instance, to calculate the number of fries sold at a McDonald’s outlet, it is easier to calculate based on the number of people who buy fries in a day, which is the demand side rather than calculating how often McDonald’s’ stock gets refilled, which is the supply side.
Abstract
These are the most notorious of all guesstimates. To solve an abstract guesstimate, you must first break the abstract problem into smaller guesstimates, which would fall into either of the two types mentioned above and follow the method highlighted under each type.
For instance, if you are asked to calculate the number of leaves in a forest. First, calculate the number of maximum trees possible in 1 sq km of area and, based on that number, you can figure out the number of trees in the forest. Thereafter, to count the leaves, estimate the number of leaves on a Bonsai (small tree) tree, possibly at your home, and scale it to the size of a real tree and increase the number of leaves accordingly.