What kind of sample is best for external validity?
To ensure high external validity, it’s important to draw a sample that’s representative of the population you want to generalize to. It’s always best to choose a probability sampling (also known as random sampling) method for this.
The most popular sampling methods are stratified sampling, systematic sampling, simple random sampling, and cluster sampling.
A probability sampling method also increases other types of validity, such as internal validity, and it reduces bias.