Information from all past experience can be divided into two groups:
In many cases the factors causing the unwanted variation are unknown and must be inferred from the data.
Noise can be seen as the result of:
The noise tries to be represented by calculating the prediction error
All information got random noise that is related to the data collection process.
Example: reading of GPS 'jump around' though always remaining within a few meters of the real position.
see Statistics - Bias (Sampling error)