# Bayes theorem probaility doesn't make sense

I try to use Bayes Theorem to calculate the probability of $$P(A|B)$$. I have $$P(A)$$ in column1, $$P(B|A)$$ in colmn2, $$P(B)$$ in column 3. I get the following:

my calculations were:

$$P(B/A) = 0.8\times A$$ $$P(B) = (Bx*0,55)+((1-Bx)*(0,55))$$ $$P(A/B) = (Ax*Bx)/Cx$$

The probability gets above 1. What am I doing wrong?

• What is $Bx$, $Ax$, and $Cx$? – OmG Nov 15 '19 at 13:47
• The columns a=column1, b column2, etc.. x indicates the element per row. – havaey Nov 15 '19 at 13:54
• Then your data in the last two rows is obviously wrong! It's simply impossible for P(B | A) * P(A) > P(B) – ManRow Nov 28 '19 at 11:45

The problem is your data! For example, the last row shows that $$\mathbb{P}(A) = 1$$ and $$\mathbb{P}(B|A) = 0.8$$. If $$\mathbb{P}(A) = 1$$ means $$A$$ is equivalent to all possiblities of event world. Hence, $$\mathbb{P}(B|A)$$ couldn't be anything except 1.
Bayes Theorem or not, you simply cannot have $$P\left(B\,|\,A \right) \times P\left(A\right) > P\left(B\right)$$ but this "impossibility" is exactly what happens in your last two rows!!