sampling methods

random (qual/quan)

characteristics

quan

Every member of the target population has an equal chance of being selected and participants can be selected by drawing names from a hat or allocating everyone a number and then using a form of computerised random number generator to select the numbers.  

A random sample should give a representative sample as the randomisation should ensure that the sample is not biased in any way. It should be possible to generalise from a random sample to the rest of the target population.

convenience (qual/quan)

characteristics

quan/qual

Opportunity sampling involves selecting those people who are readily available to participate. The sampling method is quick, easy and convenient as you simply choose those people who are around at the time the researcher wishes to do the study.    

Often psychology lecturers use students in their classes to take part in their research and often university students have to participate in a certain number of studies to gain credits for their degree, providing a never-ending stream of participants for their opportunity samples. This sample technique is a form of non representative sampling, as it cannot be be guaranteed that those who are available will match the target population.

volunteer (qual/quan)

characteristics

quan/qual

A volunteer sample means that researchers advertise for participants by placing notices, posters and adverts in places where potential participants might see them, often in newspapers or nowadays on social media. Participants who are interested then get in touch with the researcher in order to make contact and see whether they are suitable to take part.

People who volunteer often have a vested interest in the line of research being under-taken and often share certain similarities in terms of their personalities, life-styles, educational and socio-economic backgrounds. This can lead to self selection bias and is a form of non representative sampling.

purposive (qual/quan)

characteristics

qual

Participants are chosen because they possess specific characteristics that make them suitable for the research project.  Sometimes people are handpicked to get a more diverse set of participant for the study and tis is called maximum variation type; for example an acculturation researcher might seek try to get a diverse sample including refugees, immigrants, sojourners and international students

Researchers may also attempt to get a more homogenous sample, meaning they carefully select people who all have share very similar experiences and/or have similar  backgrounds in order to get a more representative  view of that ‘type’ of person; this is known as ‘homogenous type’ sampling.

snowball (qual/quan)

characteristics

qual

This sampling method involves asking people who are already part of the sample and have been chosen via some other means, e.g. volunteer or opportunity sample, whether they can think of anyone they know from their own social circle, who meets the criteria and might like to become part of the study. This is particularly useful when there is no sampling frame.

It's often used when the target group is small, hidden, or difficult to access (e.g., drug users, refugees, niche professionals).