sampling types - Math

Simple random

selecting a sample completely at random, equal chance of choosing. Eg. out of a hat, random number generator.

Convenience

getting data by selecting people who are easy to reach. Eg. Ask friends, first 5 people. Not random so could be bias and unrepresentative of the population.

Systematic

choosing a random starting point and using a fixed interval. Eg. every 3rd student in a class, starting at a random point and taking every fifth entry.

Quota

Sample sizes in proportion to who is being sampled. Eg. the school has 250 boys and 300 girls so the cafeteria manager decides to interview 25 boys and 30 girls.

Stratified

Divide the population into smaller groups (strata) with shared characteristics then take samples from each group proportionate to the amount of people. Eg. If theres more blonde people than red heads more blonde people should be surveyed.

Difference between quota and stratified

Quota sampling is based on a predetermined number of units chosen in a non-random manner while stratified is drawing a random sample from each subgroup