# Porbability, not sure I get it

1. Dec 19, 2012

### liquidFuzz

An example of what I try to calculate. In a string matching between two strings the whole strings aren't compared, just a section.

A = The probability of a true hit.
A true hit being:
PHP:
B = The probability of artifact being correct.
Falls artifact being:
PHP:
I though I'd use Bayes theorem to calculate the probability, but I'm not sure of how to do it.
$P(A|B)\frac{P(B|A)P(A)}{P(B)}$

In my example A is much higher than B, would this imply:
$P(A|B)\frac{P(≈A)P(A)}{P(B)} ≈ P(A)$

2. Dec 19, 2012

### Staff: Mentor

I don't get what you want to calculate.

3. Dec 19, 2012

### liquidFuzz

A = Probability of something happening
B = Probability of something goes wrong if A happened.

4. Dec 19, 2012

### Staff: Mentor

That is not the problem.
What do you want to calculate, where do those probabilities come from, and so on?

So P(B) is P(goes wrong|A)?

5. Dec 22, 2012

### liquidFuzz

Ok, I snitched the text that got my puzzled. Please comment!

#### Attached Files:

• ###### Bayes.png
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6. Dec 22, 2012

### chiro

Did you have a particular question in mind given your snitch?

7. Dec 22, 2012

### liquidFuzz

Yeah, I don't quite get the part when he or she goes all P(A|B) and P(B|A).

This is a part of a report that I'm helping to proofread.

8. Dec 22, 2012

### Staff: Mentor

Probability values above 1 do not make sense. In addition, the text does not state clearly where probabilities come from. It is like "probability of an apple" - sure, I know apples, but an apple where, in which setup?

9. Dec 22, 2012

### liquidFuzz

I guess I'll get away with just commenting this. "Vague reasoning."

Thanks!