How do we select from the population what goes into our sample? Before we answer this question, we must first decide how we're going to represent different sample and population sizes.
Mathemtically, A capital N is used to represent the size of a population.
A lowercase n is used to represent the size of a sample.
Letís say you want to find the average GPA of a student at your university. Your university has 20,000 students, and you select 100 students and ask them their GPAs.
What are N and n in this example?
N, the size of your population, is 20,000
n, the size of your sample, is 100
Samples are drawn from populations through several different sampling methods:
|Simple Random Sampling|
Every member of the population(N) has an equal chance of being selected for your sample(n).
This is arguably the best sampling method, as your sampleis almost guaranteed to be representative of your population. However, it is rarely ever used due to being too impractical.
With this method, the population(N) is split into non-overlapping groups ("strata"), then simple random sampling is done on each group to form a sample(n).
One example of this would be splitting a population of students into men and women, then sampling from each of the two groups. This may allow us to collect the same amount of information as simple random sampling, but use less people.
In this method, every nth individual from the population(N) is placed in the sample(n).
For example, if you add every 7th individual to walk out of a supermarket to your sample, you are performing systematic sampling.
Easily obtained individuals from the population(N) are placed in the sample(n).
Simply put, in this type of sampling you pick the easiest way of getting your sample. This type of sampling is sometimes called voluntary response sampling, because individuals often select to be a part of the sample. This can be a problem, because there may be a difference between people who choose to participate and people who donít.
While there are other sampling methods not mentioned in this article, these are the most commonly used.