Statistics is usually defined as the science of the methods of data collection and analysis (often called data science these days), but from a somewhat more conceptual perspective, statistics is also the science of methods dealing with uncertainty. The sources of uncertainty in statistics may be divided into two groups. First, uncertainty associated with the actual data one has and, second, uncertainty associated with respect to the mechanism which produced the data. These kinds of uncertainty are often interrelated in practice, yet it is useful to distinguish them.