When doing research on a large population, it is often impractical to study every single member of the group. Instead, we look at a portion or sample of the population. Ideally, Probability Sampling Methods are used, where participants are randomly selected and every member of the population has an equal chance of being selected. When it is impossible or impractical to use probability sampling, researchers use Nonprobability Sampling techniques, where participants are selected deliberately, by convenience, or haphazardly.
Nonprobability Sampling is commonly used in studies where the trait or behavior being investigated occurs so rarely in the general population that it becomes impractical to use a random selection process. For example, if you wanted to study Savant Syndrome, a condition where individuals with developmental disorders exhibit remarkable ability in specific areas like the autistic character in the movie Rain Man, who was a genius at calculations. It would be very difficult and expensive to go around the world and randomly select subjects from the small population of savants. In this case, the best route to take would be to contact as many savants as you could find in your state and neighbouring areas.
The advantage of using Nonprobability Sampling is it saves time and cost, while allowing you to closely investigate the syndrome. The disadvantage is that you will not be able to make broad generalizations about the entire population of people with the condition.