Census vs Population
Both terms below come up in the same part of the course, and students mix them up. Here is each one defined on its own, side by side, so you can see where they part company.
Census
Collecting data and study design
A census is the collection of data from every single member of a population rather than from a sample.
In a census you attempt to measure the whole population, so the values you compute are parameters rather than estimates. For example, asking every employee at a 40-person company their salary is a census of that company. A census removes sampling variability, but for large populations it is often too expensive or slow to carry out. When a full census is impractical, you take a sample instead.
Population
Collecting data and study design
A population is the entire group of individuals or objects you want to study and draw conclusions about.
A population includes every member of the group in question, not just the ones you happen to observe. For example, if you want the average height of all students at a school, the population is all of those students, and its true mean height is a fixed number you usually cannot measure directly. A numerical fact about a population, such as the population mean (the Greek letter mu, written ), is called a parameter. You typically estimate it from a smaller sample.