Why do we take samples?
You want to understand certain things and have some objective in mind. In each case there is a target population.
The goal for many research projects is to know more about your objective, i.e., your population. This is what you are interested in. For instance, if you were a conservation officer you might be interested in the number of deer in central Pennsylvania. In this case, you have a certain goal in mind. What steps can we take to understand the population better?
What we can do is to take a sample. However, the difficulty and the one major objective in statistics that now arises is inference.
One important objective of statistics is to make inferences about a population from the information contained in a sample.
We should always keep in mind that we perform sampling because we want to make this inference. Because of this inference we begin to talk about things like confidence intervals and hypothesis testing. A good picture to represent this situation follows: