Updated Nov 2019.
Understanding sampling methods by just reading definitions can be difficult. The best way to learn about them is to see them in action. The following simple activity has been designed to help.
What you need to know…
Here are four of the common sampling methods used in quantitative research. You need to know what these mean and the difference/s between them. If you need to revise the definitions, use the textbook to help you (pp330-335).
*These are defined at the bottom of these page if you want to read up about them.
- Lesson Idea: Understanding Sampling Methods
- Lesson Idea: Sampling Methods – Practice for Paper 3
- Lesson Idea: Understanding Self-selected Sampling
Read the following summaries of how the researchers got their participants and identify which sampling method was used.
#1 A researcher is investigating Japanese citizen’s responses on pubic trains as someone collapses and needs help.
#2 In a study on what will happen when smoke fills a room while people are sitting in it, the researchers wanted to compare medical students and history students. They sent a google form in an email to all registered medical and history students asking for participants.
#3 While researching the amount people donate to charities, researchers stood on the sidewalk and asked participants who walked by if they wanted to participate.
#4 When studying homosexuality in identical twins, researchers posted an advertisement in a popular magazine for gay men asking for participants. Those who were interested responded to the advertisement.
#5 When studying the effects of a terrorist attack on London city dwellers, researchers used city council data to get the names and addresses of all London citizens. They used a random number generator to phone and ask for participants.
#6 When studying the effects of compliance techniques, researchers walked the streets of Los Angeles neighbourhoods an asked for volunteers to participate in their study by putting big signs saying “Drive Safely” in their front yards.
#7 In a study comparing London and Taxi bus drivers, the researchers wanted to compare the differences in their brains. So they asked the bus and taxi companies to ask their employees if they’d like to participate. Those that did want to participate were told to contact the researchers.
There are two sampling methods commonly used in qualitative research: snowball sampling and purposive sampling. Let’s start with the definitions and then see if you can spot them above.
Snowball sampling: is a type of non-probability, convenience sampling. If we say something is “snowballing,” it means it’s gradually getting bigger, like a cartoon image of a snowball rolling down a snowy mountain and gradually increasing in size. This is how a snowball sample works: the sample size gradually increases as a small group of initial participants invite others to take part in the study. These initial participants who ask others to join the study are called seeds. When new participants are found, they are also encouraged to recommend others to participate. This continues until the desired sample size is reached. This is an especially effective method when the participants are hard to find or do not want to be found (what researchers call “hidden populations”).
Purposive sampling: If something is purposive it means it’s done with a purpose, so when purposive sampling the researchers deliberately (and purposefully) select and recruit participants for their study because they possess particular characteristics that are of interest to the researchers. This recruitment may happen through contacting participants directly, through advertising, or perhaps by having possible participants referred. And in fact, a purposive sample may have an element of snowballing: researchers may use seeds that have particular characteristics and instruct them to ask others who share those same characteristics.
Note: Sometimes there could be more than one possible answer as it’s not always clear-cut which method has been employed. What is important is that you can define the sampling method and give an explanation that shows how the example above is an example of that particular method.
Travis Dixon is an IB Psychology teacher, author, workshop leader, examiner and IA moderator.