Welcome everybody to a new round of pattern recognition question and answers.

And today we want to have a short chat about measures of evaluation classification systems

and how a classification system can be altered in order to change the evaluation outcome.

So I think you will find this interesting and I also brought a very interesting example

to discuss the ideas of classification systems.

Okay, so here's what I have for you.

Today we want to discuss another round of question and answers and in particular we

want to talk about the questions that you had.

You sent them by email or posted them in the forum.

And the key question that came up lately was can you discuss somehow how do I evaluate

a classification system.

So we talked about this in the lecture but I found that we need some additional explanation

to actually understand what the classification systems do and how we can evaluate them.

So of course we will discuss this in an example.

Everybody is talking about diseases nowadays and it seems everybody is becoming an expert

on disease classification.

So I also thought it would be interesting to talk about a typical disease and I took

the zombie disease.

So I think this is one of the most terrifying things that could happen to the world.

So I think this is the right time to discuss what we should do when we encounter the zombie

disease.

So you see here this is how we can evaluate then the outcome of a test.

So we're preparing already a test system to figure out who's a zombie and who isn't.

And we would of course then have a couple of possible outcomes.

And you know people could actually be zombies so that would be the reference.

So these are the columns or they could not be zombies.

So they would be either positive or negative.

And then of course our classification result the hypothesis could then have one of the

two outcomes.

So this would then also be either positive or negative.

And of course there is regular humans.

So this is a true negative.

They are just regular humans and of course the test result should also be negative for

humans.

Then of course we want our classification system to actually detect zombies.

So there are factual zombies out there that we want to detect and we want the result to

be also zombie.

So this would be a true positive.

Now of course our classification system can also make mistakes.

And then you know sometimes we detect somebody as a zombie who isn't actually a zombie.

And I brought this small example here that you can memorize this.

So this is from actually a very interesting horror movie, Shaun of the Dead.

And in the scene here Shaun had a really bad hangover and he just goes out to the shop

to buy a drink.

And you see he's so severely hungover that he first of all he doesn't realize or the

walking dead around him and second they don't recognize him because he behaves like one

of them.

So yeah this would be then a false positive.

So people and luckily here also walking dead zombies think you are one of them.

And then there's also the other case that we somehow miss that somebody is dead and