sampling selection and techniques - RM

• Random SamplingRvery person in a given target population has an equal chance of being selected.  This means that it is necessary to have a list (sometimes referred to as a sampling frame) that identifies every person in the target population eg imagine you wanted to take a sample of people from Harrow Swimming Club (your target population). You would need to get every name of every member and then take your sample. In this case you could get all their names and put them in a hat, then pull out each one. Every member of the swimming club has an equal chance of being selected.        

          
• 1. Random SamplingPopulation validity This is a representative sample and has high population validity.

   

  • Weakness Selecting a random sample does not guarantee a sample that is totally representative of the population concerned.  However, as long as the target population and sample size have been chosen carefully, the laws of probability predict the chance of selecting a biased sample is minimal.             
• 2) Opportunity SamplingThis is a widely used non-random method of sampling because it is convenient (in fact it is sometimes referred to as a convenience sample).  It is when you use people who are available to you. To select people who have the opportunity to take part. This can be for many reasons, i.e. they know you, they just happen to be in a particular place eg In the swimming club example, you could stand at the entrance of the building and ask members as they come in whether they want to participate in your study. They ‘happen’ to be there.
• 2) Opportunity Sampling - Weakness – population validity

  Its main weakness is that it is unlikely to generate a sample that is representative of the wider target population from which it is drawn. Therefore the findings of the study cannot be generalised.  This would therefore have low population validity.
• 3) Volunteer Sampling

   

  Participants select themselves to take part in a research study, often replying to an advertisement.  This type of sampling has been widely used in university research. For example in the swimming club example above you could place an advertisement in the entrance to the swimming pool asking if anyone would be interested in being a participant in your research

   
• 3) Volunteer Sampling

   Population validity - Therefore, the data gathered from this potentially biased sample are unlikely to be representative of the target population and so the findings of the study cannot be generalised.  This would therefore lead to low population validity..

   

   Weakness - The majority of a given target population are unlikely to respond to the request to participate, and those who do respond (i.e. volunteers) may not be typical of the target population in some way. 
• 4) Stratified Sampling

  If the target population has prominent characteristics that need to be proportionately represented in the sample recruited, a stratified sampling technique can be used.
• 4) Stratified Sampling

   

  For example if you are researching stress in the workplace in a company, you can find out how many staff occupy different roles within the company for example office clerks, managers, canteen staff, cleaners etc. As there may be more clerks than managers, more clerks need to be recruited for the study than managers in order to represent the company staff more fairly. Each subgroup within the company can be randomly sampled by placing the names of all the clerks in a hat, for example, and drawing out a proportionate number.
• 4) Stratified Sampling

   

  • Weakness It can be difficult to know how many of each group to choose in order to make sure that the findings are generalisable. It is difficult to know how many individuals make up an appropriate sample; with stratified sampling, where the numbers in each group may be small, it is harder.