Convenience Sample vs Simple Random Sample
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.
Convenience sample
Collecting data and study design
A convenience sample includes whichever individuals are easiest to reach, rather than selecting them through any random process.
A convenience sample takes the members who are simplest to access, so it usually fails to represent the population and tends to be biased. For example, surveying only your own friends about a school policy is a convenience sample. Because the selection is not random, there is no way to know the direction or size of the bias, and standard error formulas do not apply. Ease of collection does not make up for a lack of randomness.
Simple random sample
Collecting data and study design
A simple random sample is selected so that every possible sample of the given size has an equal chance of being the one chosen.
In a simple random sample, often shortened to SRS, each individual and each group of individuals is equally likely to be picked. For example, numbering all 500 students at a school and using a random number generator to select 30 of them produces an SRS. This equal-chance selection removes selection bias and is what justifies the standard error formulas used later in inference. It usually requires a list of the whole population to sample from.