First, let define and distinguish the terms Population, and Sample

If the data consist of all possible observations of a certain phenomenon, we call it Population

If the data contains only a part of these observations, we call it Sample

The main reason to analyze a sample to reach a conclusion on a population is straightforward

- Analyzing a population would demand massive amount of time, resources, and money!

Example: Suppose that we want to know the Mean weekly income of all households in Europe - That would be pretty impossible
- Let draw a sample of 2000 households, and calculate the mean weekly income as follows