Lecture 7 Selecting Samples

1. Lecture 7Selecting Samples

2. Selecting samples Population, sample and individual cases Source: Saunders et al. (2009) Figure 7.1 Population, sample and individual cases

3. The need to sample Sampling- a valid alternative to a census when • A survey of the entire population is impracticable • Budget constraints restrict data collection • Time constraints restrict data collection • Results from data collection are needed quickly

4. Overview of sampling techniques Sampling techniques Source: Saunders et al. (2009) Figure 7.2 Sampling techniques

5. Probability sampling The four stage process • Identify sampling frame from research objectives • Decide on a suitable sample size • Select the appropriate technique and the sample • Check that the sample is representative

6. Identifying a suitable sampling frame Key points to consider • Problems of using existing databases • Extent of possible generalisation from the sample • Validity and reliability • Avoidance of bias

7. Sample size Choice of sample size is influenced by • Confidence needed in the data • Margin of error that can be tolerated • Margin of error (also called The confidence interval ) is the plus-or-minus figure usually reported in newspaper or television opinion poll results. For example, if you use a margin of error of 4 and 47% percent of your sample picks an answer you can be "sure" that if you had asked the question of the entire relevant population between 43% (47-4) and 51% (47+4) would have picked that answer. • Types of analyses to be undertaken • Size of the sample population and distribution

9. The importance of response rate Key considerations • Non- respondents and analysis of refusals • Obtaining a representative sample • Calculating the active response rate • Estimating response rate and sample size

10. Selecting a sampling technique Five main techniques used for a probability sample • Simple random • Systematic • Stratified random • Cluster • Multi-stage

11. Simple random sampling • Number each of the cases in your sampling frame with a unique number. • Select cases using random numbers until, actual sample size is reached. • Computer aided telephone interviewing (CATI) software

12. Selecting a probability sample - diagram

13. Systematic Random Sampling • Number each of the cases in your sampling frame with a unique number. • Select the first case using a random number • Calculate the sampling fraction • Select subsequent cases systematically using the sampling fraction to determine the frequency of selection. • Sampling fraction = actual sample size/ total population

14. Example

15. Stratified random sampling • Choose the stratification variable or variables • Divide the sampling frame into the discrete strata. • Number each of the cases within each stratum with a unique number • Select your sample using either simple random or systematic random sampling

16. Cluster sampling • Choose the cluster grouping for your sampling frame. • Number each of the clusters with a unique number. • Select sample of clusters using random sampling

17. Multi-stage sampling

18. Illustration of four phases - process

19. Non- probability sampling (1) Key considerations • Deciding on a suitable sample size • Data saturation • Selecting the appropriate technique

20. Non- probability sampling (2) Sampling techniques • Quota sampling (larger populations) • Purposive sampling • Snowball sampling • Self-selection sampling • Convenience sampling

21. Selecting a non-probability sampling technique

22. Impact of various factors on choice of non-probability sampling techniques

23. Quota Sampling • Divide the population into specific groups. • Calculate quota for each group based on relevant and available data • Collect data from each quota

24. Purposive sampling • Extreme case/deviant sampling: unusual or special case enable to learn the most about the RQ. • Heterogeneous or maximum variation sampling: representing different subgroups • Homogeneous sampling: One subgroup. • Critical case sampling: • If it happen there, it will happen everywhere.

25. Snowball sampling • Make contact with one or two cases in the population. • Ask these cases to identify further cases. • Ask these new case to identify further new cases. • Stop when either no new cases are given or the sample is large enough.

26. Self select sampling • Publicize your need for cases • Collect data from those who respond

27. Haphazard sampling • Also called purposive or availability sampling. • Select case based on ease or convenience.

28. Impact of various factors on choice of probability sampling techniques

29. CHECKLIST - Selecting your sampling frame • Are cases listed in the sampling frame relevant to your research topic, in other words will they enable you to answer your research question and meet your objectives? • How recently was the sampling frame compiled, in particular is it up to date? • Does the sampling frame include all cases, in other words is it complete? • Does the sampling frame exclude irrelevant cases, in other words is it precise? • Can you establish and control precisely how the sample will be selected?

30. CHECKLIST - Using sampling as part of your research • Consider your research question(s) and objectives. You need to decide whether you will be able to collect data on the entire population or will need to collect data from a sample. • If you decide that you need to sample, you must establish whether your research question(s) and objectives require probability sampling. If they do, make sure that a suitable sampling frame is available or can be devised, and calculate the actual sample size required taking into account likely response rates. If your research question(s) and objectives do not require probability sampling, or you are unable to obtain a suitable sampling frame, you will need to use non-probability sampling. • Select the most appropriate sampling technique or techniques after considering the advantages and disadvantages of all suitable techniques and undertaking further reading as necessary • Select your sample or samples following the technique or techniques as outlined in this chapter • Remember to note down the reasons for your choices when you make them, as you will need to justify your choices when you write about your research method.