Can you absolutely trust the data you are gathering? Why?
Answer
No, because data can contain errors, some that can be very subtle. That is why you need to work on maintaining and improving your data. Also, as a person subjected to the workings of the algorithms, very often you will not be able to learn what was the data that the agorithm made its decision on (and thus you will not be able to verify if that data is correct or not)!