Sampling Methods

• Sampling = The method used to select participants or to sample behaviours in an observation.
• Opportunity Sample

  • A sample of participants produced by selecting people who are most easily available at the time of the study.

  Method:

  • Recruit those who are most convenient or most available (e.g. people walking by you in the street )

  Strengths:

  • Easiest method as you use the first suitable participants you can find.
  • Takes less time to find your sample

  Weaknesses:

  • Biased because the sample is drawn from a small part of the population.
• Random Sample:

  • A sample of participants produced by using a random technique so every member of the target population has an equal chance of being selected.

  Strengths:

  • Unbiased - All members of the target population have an equal chance of being chosen.

  Weaknesses:

  • Need to have a list of all members of the population and then contact all members that are selected. - time consuming.
• Random Techniques:

  Using the Lottery Method:

  1. Obtain a list of all the people in the population
  2. Paul all the names in a lottery barrel or hat
  3. Select the number of names required
• Random Techniques:

  Random Number Table:

  1. Every member of the population is given a number.
  2. The starting position in the table is determined blindly by placing your finger anywhere.
  3. If your population is less than 100 you only need 2 digit numbers so read the table 2 digits at a time.
• Random Techniques:

  Random Number Generator:

  1. Number every member of the population
  2. Use a software (e.g. Microsoft Excel -> Type RAND=(100)) or use an app to get a random number between 1 and 100 (if you have a population of 100).
• Stratified Sample:

  • A sample of participants produced by identifying subgroups according to their frequency in the population. Participants are then selected randomly from the subgroups.

  Method:

  1. Subgroups (or strata) within a population are identified (e.g. boys and girls or age groups).
  2. Participants are obtained from each of the strata in proportion to their occurrence in the population.
  3. Selection from the strata is done using a random technique.
• Stratified Sample:

  Strengths:

  • Likely to be more representative than other methods due to the proportional and randomly selected representation of the subgroups.

  Limitations:

  • Very time consuming to identify subgroups, randomly select participants and contact them.
• Systematic Sample:

  • A sample obtained by selecting every nth person (where n is any number). This can be a random sample if the person is selected using a random method. Then, you select every nth person.

  Method:

  1. Use a predetermined system to select participants (e.g. select every 14th person) from a list. Apply the numerical interval consistently (e.g. choose every 14th throughout).

  Strengths:

  • Unbiased as participants are selected using an objective system

  Limitations:

  • Only unbiased/random if you select a number using a random method and start with this person.
• Volunteer Sample:

  • A sample of participants that relies solely on volunteers to make up the sample. This is also called a self-reflected sample.

  Method:

  Advertise in a newspaper, notice board or on the internet for example.

  Strengths

  • Gives access to a variety of participants which may make the sample more representative and less biased.

  Limitations:

  • Biased as participants are likely to be more highly motivated and/or with extra time in their hands. They might be more highly motivated to be helpful or they may need the money offered for participation - results in volunteer bias
• Volunteer bias = A form of sampling bias because volunteer participants have special characteristics. (e.g. being more motivated than randomly selected participants.
• Bias = A systematic distortion.
• Generalisation = Applying the findings of a particular study to the population.
• Population = The group of people that the researcher is interested in. The group of people from which a sample is drawn.
• Sampling = The method used to select participants or to sample behaviours in an observation.