What is sample and population?
When you measure something and get your data, what you get is called sample from the entire population of your data. For example, you have some questionnaire survey about marketing of a product. You ask many questions especially whether people like to buy this new product. If you can distribute your questionnaire to all citizen of the city, and all of them response to your questions, you get the population. In reality, it is not practical to distribute questionnaire to millions of people. What you can do is take some random sample, say a hundred or a thousand, from the population and assume that your sample is a valid representation of the entire population. From your sample, you can estimate the true value of the population.
