How To - Bayes Theorem

I don't know about you but when the Bayes Theorem came up first in my class - I sucked at it.
Frankly I was too lazy to look into it, well at first I really tried and thought I got it.
But now I totally got it... and it makes so much sense now, I would like to beat my older me with a tennis racket -__-

So here we go, 10min tops - if you are one of the slow ones like me

In this tutorial we will use the Bayes Theorem to compute one of the basic exercises in statistics.
Seriously, this case study comes up every time ._.

A Zombie Virus was leaked and the World Health Org has created a test that can identify infected citizens - as you know, it is highly desirable to identify the right people and protect the healthy ones...
Ca. 5% of the population have the Virus.
The test has a positive success rate of 85%, correctly identifying sick people. Only in 10% of the cases, the test identifies people as sick, although they are healthy.

What we want to know is, how high are the chances, that you are sick - IF the test says you have the virus?
Because if I were you, I'd not want to turn into a Zombie. And If you are great at calculating in your head, you have a shot on finding out if you are prone turn - before you get shot. Like, in the head and stuff.

The simplest way to fill in the Bayes Formula, is with the help of a confusion matrix. Don't worry, that's just the name and some terrible sense of humor of the guy who invented the confusion matrix.

As you can see we put the values that we had from our text (green) into the confusion matrix and derived the values for the rest of the population (red) from those, which in total add up to 100%
(i.e. 5% of the people are sick --> the rest must be healthy, so 95% is not sick)

Now that we have all additional values that we need, we can start filling in the Bayes Theorem:

Congratulations, you have a chance of 31% that you might turn into a zombie.
For me personally, anything that is less than 49% is healthy - you might want to start running though, in crisis situations the WHO might still want to shoot you.
After all you never can know for sure... until it's too late, right?

In anticipation of my favourite Zombie series^^ Resident Evil Retribution 2 , Retribution brought back true horror to the series in my eyes - what about you? Any zombie fans out there?


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