Sampling
Session Objectives By the end of the session you will be able to: Explain what sampling means in research List the different sampling methods available Have had an introduction to confidence levels and confidence intervals
Sampling Sample frame = the target population you wish to research Census = all the respondents in the sample frame participating in the research Sample = a smaller group selected from the sample frame to participate in the research Sample method = the technique used to select the sample
Representativeness The aim of any sample is to represent the characteristics of the sample frame. There are a number of different methods used to generate a sample. As a researcher you will have to select the most appropriate method meet the requirements of your research.
Sampling Methods Task 10 mins Small groups Match up the type of sampling with the descriptions
Sampling Sampling methods can be split into two distinct groups: 1. Probability samples 2. Non-probability samples
Sampling Probability Samples Probability samples offer each respondent an equal probability or chance at being included in the sample. They are considered to be: Objective Empirical Scientific Quantitative Representative
Sampling Non Probability Samples A non probability sample relies on the researcher selecting the respondents. They are considered to be: Interpretivist Subjective Not scientific Qualitative Unrepresentative
Probability Sampling Methods Random Sampling Systematic Random Sampling Stratified Random Sampling Cluster Random Sampling Quota Random Sampling Multi-Stage Sampling
Random Sampling This involves selecting anybody from the sample frame entirely at random. Random means that each person within the sample frame has an equal chance of being selected. In order to be random, a full list of everyone within a sample frame is required. Random number tables or a computer is then used to select respondents at random from the list.
Systematic Random Sampling This selection is like random sampling but rather than use random tables or a computer to select your respondents you select them in a systematic way. E.g. every tenth person on the college list is selected.
Stratified Random Sampling An appropriate group is decided upon i.e. female, male, 16 18 year olds and the participants are picked randomly from within the strata
Cluster Random Sampling Similar to stratified sampling but the groups are selected for their geographical location i.e. school children within a particular school. The school is the cluster with the children being selected randomly from within the cluster
Quota Random Sampling Having decided on the characteristics of the sample frame, a sample is selected to meet these characteristics. E.g. if the sample frame is car drivers and the car driving population is 55% male and 45% female then the quota would require the same proportions. Participants would be selected to fill this quota using the random method
Non-probability Sampling Convenience Sampling Snowball Sampling These non-probability methods can be used in conjuncture with the cluster, quota or stratified methods, however they will remain non-probability samples
Convenience Sampling This involves selecting the nearest and most convenient people to participate in the research. This method of selection is not representative and is considered a very unsatisfactory way to conduct research.
Snowball Sampling This type of sampling is used when the research is focused on participants with very specific characteristics such as being members of a gang. Having identified and contacted one gang member the researcher asks to be put in touch with any friends or associates who are also gang members. This type of sampling is not representative however is useful, especially where the groups in the research are not socially organised i.e. they do not have clubs or membership lists.
Sampling Methods Task 10 mins Small groups Return to the task and see how many you got right and put right any you got wrong.
Quantitative Research - Sample Size When conducting probability sampling it is important to use a sample size that is appropriate to the aims and objectives of the research. There are tables recommending sample size (see de Vaus, 1996 pp 71-72) but as a general rule the smaller the total sample frame the larger the sample ratio needs to be. A common error is to assume that the sample should be a certain percentage of the population, for example 10%. In reality there is no such relationship and it only the size of the sample that is important. A probability sample size of 100+ is considered a large enough sample to conduct statistical analysis
Statistics and Samples When presenting your research you need to be able to demonstrate, how representative of the whole population the sample data you have collected is. There are two statistical test used to do this: Standard Error Confidence Levels
Standard Error Using the standard deviation of the population and the sample size a statistical calculation can measure the degree of error likely to occur between the results of a sample and the results of a census, this is call the standard error. The larger the sample the lower the standard error. When a probability sample of 100+ is undertaken the distribution can usually be assumed to be normal When the sample has normal distribution, we can use the z score approach to obtain confidence limits for the sample mean.
Confidence Levels Confidence levels are calculated using the Central Limit Theorem Using this and the sampling error we can then use the area below the normal distribution curve to make predictions about our sample. As well as making predictions we can use the properties of the normal distribution curve to provide us with confidence levels There are three confidence levels 68%, 95% and 99%
Confidence Levels The concept does not mean that we are 95% sure that a single sample mean lies within these limits. The 95% confidence limits mean that if we drew many samples, and find the mean for each, then we can expect 95% of the sample means to lie within the stated limits. 95% confidence is considered acceptable in social research, medical research often requires 99% confidence
Confidence Levels Bell Curve
Review Can you explain what sampling means in research? Can you list the different sampling methods available? Have had an introduction to confidence levels and sample error?
Further Reading Drummond, A. (1996) Research methods for therapists. Cheltenham, Nelson Thornes Fielding J and Gilbert N (2000) Understanding social statistics London: Sage Thomas J R and Nelson J K (2001) Research methods in physical activity 4th Ed, Leeds, Human Kinetics Trochim W (2007) available at www.socialresearchmethods.net/kb/sampstat.php