How can I calculate the error rate?

Hi, I have two sets of variable. A is real world data and B is my predictions. The question is how can I calculate the error rate?

For example;

A=[30,25,46,20,18,12,...47]

B=[28,20,46,25,30,12,...44]

Answer 1

[David Sanchez]

error_rate = mean(A-B)

[net]

Thanks, for your help. It works great but also I need mathematical definition about it. I will more happy if you can give me links about the topic. I must give reference to use it.