Selection of the data may be biased, and if so, the results will not be fair. For example, trying to find out how many people like fish near a fish market would not be the same, as asking people in a more neutral area. Data could be missing. Censored data points are those whose measured properties are not known precisely, but are known to lie above or below some limiting sensitivity. Truncated data points are those which are missing from the sample altogether due to sensitivity limits.
Artifacts can enter into an experiment due to lack of controls.
In some cases, they can be traced back to the experimental design itself. In other cases, they can be traced back to how the procedure was followed. Research involving humans introduces sometimes a complex relationship between the researchers and the subjects. The experiment is designed to show the relationship between two or more variables. The different values of the independent variable are called levels.