Very often when we are with the family dining over the table and someone comments on any topic, you’ll hear my father say “give me a number… come on, you should be able to tell me an order of magnitude”. This is what is called a “guesstimate”, from the Wikipedia:
“Guesstimate is an informal English word derived from guess and estimate, first used by American statisticians in 1934 or 1935. It is defined as an estimate made without using adequate or complete information, or, more strongly, as an estimate arrived at by guesswork or conjecture.”
In this post I just wanted to share a “number”, a guesstimate, I worked while in Egypt.
When we left our stuff in the cruise boat, Luca and I were told by our guide that there were 300 such boats cruising the Nile. We certainly could appreciate that there were many in the harbour, and later we could see the traffic through the river. On one of the conversations we started to build our guesstimate: Can we guess how many tourists come to Egypt every year? If there are 300 boats at any moment, if in our boat there are 21 + 19 + 10 double rooms, if… if…
Here it is:
- 300 boats at any moment in the Nile.
- If our boat was the average boat size: 50 double rooms, thus 100 tourists per boat.
- If 90% occupancy rate was the average during high season (4 months?); 70% during low season (8 months).
- If 4 days was the average stay of tourists in the boat:
- This is 30 rotations during high season; 60 rotations during low season
- Assuming that 50% of tourists don’t go to Luxor & Aswan, just stay in Cairo, Alexandria, Sharm el-Sheikh…
- Assuming that of the remaining 50% that go to Luxor, 30% connect between cities in other modes of transport different from boat (plane, bus, taxi, train…)
My guess is that there would be ~ 6 million visitors to Egypt every year.
Later on, I checked with Wikipedia rankings, and the real figure is about 12 million… so my guess is just 50% of the total… way far, sure, but in the order of magnitude, thus I’m quite happy with it.
You never know, maybe the input of the 300 boats was wrong; it could be that there are really 600 boats and my guess would have been just correct! (Please, if you find out that there are indeed around 600 cruise boats in the Nile, let me know ;-))
The Blog by Javier 
last year i actually bought a book called “Guesstimation: Solving the World’s Problems on the Back of a Cocktail Napkin” (http://www.amazon.com/Guesstimation-Solving-Worlds-Problems-Cocktail/dp/0691129495), it has some interesting estimation problems like the one you just wrote about, as well as some tips to improve those calculations.
That book seems quite interesting. Let’s see when and how it’ll fall in my hands 🙂
I love guesstimations. Good work Javi.
On my Interview they told me guesstimate the number of hairdressers in Madrid, go for that. 😉
In astrophysics guessing the order of magnitude is already very accurate, specially with the error margin of your assumptions 🙂
After few days hearing on Tv news the many civil uprisings against Hosni Mubarak, I came up to realise that how lucky and percise both of you (Luca & Javier) were going to Egypt on that moment and have the chance to enjoy the beauty and the peace of the region.
Nice guesstimation question. 🙂 Guesstimate are often asked during a consulting case interview having to to with market sizing. Here you find some mock cases including amongst others guesstimates: https://www.preplounge.com/en/management-consulting-cases.php?ref=header