Proportionate sampling is a sampling strategy (a method for gathering participants for a study) used when the population is composed of several subgroups that are vastly different in number. The number of participants from each subgroup is determined by their number relative to the entire population.
For example, imagine you want to create a council of 20 employees that will meet and recommend possible changes to the employee handbook. Let's say 40% of your employees are in Sales and Marketing, 30% in Customer Service, 20% of your employees are in IT, and 10% in Finance. You will randomly select 8 people from Sales and Marketing, 6 from Customer Service, 4 from IT, and 2 from Finance. As you can see, each number you pick is proportionate to the overall percentage of people in each category (e.g., 40% = 8 people).