I was reading a book called “100 Essential Things you Didnt know you Didnt know”.

A few pages into it I read a one page chapter on a very simple way to estimate things that are yet not found which i believe classifies as a mental model - and a very practical one at that.

This mental model was used to estimate the size of unknown species and also to estimate the size of Shakespeare’s vocabulary. On a business front it seems to be used very often in oil exploration as the following line from the book suggests.

“…this type of argument can be used in many situations. Suppose different oil prospectors search independently for oil pockets: *how many of them lie unfound*…”

It has direct implications in investing and helping us avoid companies that should be avoided - changing this sentence a litte bit

“…this type of argument can be used in many situations. Suppose different investors search independently for discrepancies in the financial satements: *how many of them lie unfound*…”

The model goes like this

In the authors Phd thesis defence ( this was in the days before word processors) - two examiners went through his dissertation *independently* and both of them found typos.

Examiner A found 32 typos and Examiner B found 23 typos. 16 typos were common to both examiners i.e both of them had found the same ones. Hence, A found *16 unique typos* and B found *7 unique typos*. & *16 were common*

**The simple question was how many more typos remain unfound?**

and and the formula for estimating it is equally simple.

(A-C)(B-C)/C

Where A = Typos found by examiner A

B = Typos found by examiner B &

C = Common typos found by both examiners.

Evaluating this gives us an **estimate of the unknown of 7**

(A-C)(B-C)/C

(32-16)(23-16)/16

(16)(7)/16

112/17 =**7**

Pretty cool huh!

This essentially means that if *two bearish expert investors* - A & B ( or more ) are evaluating the same company (XY Ltd) *independently* and lets say A finds 8 valid concerns and B finds 5 valid concerns and only 1 concern is common. Then the potential unfound problems with XY Ltd is a whopping **28**

(8-1)(5-1)/1 = **28**

Now lets suppose two bullish expert investors - C & D are evaulating XY Ltd and C finds 51 awesome things & D finds 60 awesome things with XY Ltd ( you see C & D are bullish and everyone loves their find! - me included ) but 50 awesome things *are common* to both C & D. Then the potential unfound awesomeness of XYltd is just **0.2**

(51-50)(60-50)/50 = **0.2**

If A,B,C & D get together then there is a strong case for *NOT INVESTING* in XY Ltd because the potential unfound risks **(28)** far exceed the potential unfound awesomness **( 0.2)**. There is likely to be a problem with XY Ltd going forward.

I think this is a superb mental model and gives rational decision making a new dimension.

Happy Investing!