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treatment or control group. 2. Snowball sampling employs a questionnaire or a request to participate. This is sent to known contacts, who are asked to provide names of other people who will agree to participate. These, in turn, are asked to provide further names and so on. 3. Quota sampling involves the researchers identifying strata in the population and selecting a convenience sample that reflects the proportions of those strata. Thus, if a particular population is composed of 400 males and 600 females, researchers might aim to survey 40 males and 60 females. It might also be composed of 500 nurses, 250 physiotherapists and 250 doctors, in which case 50 of those chosen would be nurses, 25 physiotherapists and 25 doctors. The aim of quota sampling is to make more representative a sample that would otherwise be a convenience sample. When critiquing non-probability sampling designs in research documents, you should consider whether the researchers might have been able to select a more representative sample by using an existing sampling frame (such as doctors’ lists of patients or a national register of nurses) and employing random sampling techniques. Reading and evaluating research reports page 23 Determining a suitable sample size can be difficult. The larger the sample the more likely it is to be representative of the population from which it is drawn. However, the larger the sample the greater the resources needed for data collection and analysis. Where limited resources have necessitated the adoption of a small sample (and/or of non-probability sampling) this should be acknowledged in the research document. If a small, non-probability sample has been used, researchers should be extremely cautious about claiming that their findings can be extrapolated to the entire population. Whenever possible, and prior to sample selection, researchers should take expert advice about sample size from a statistician (this too should be noted in the research document). Exercise 8 What types of sample were used in the studies described by Allen et al. and McBride? Do you think that the sizes of the samples were adequate? Do the authors say whether they took statistical advice on sample size? Make some notes and then turn to the discussion in Section 6. Sampling in qualitative research Qualitative researchers rarely employ random sampling techniques, but neither do they claim that their data and interpretations are generalizable to larger populations. They do, however, attempt to highlight and explore issues that may be interesting and relevant to people in settings and contexts other than that in which the research was carried out. Thus, a study of the morale of staff at one hospital may be of interest to staff at a second hospital; an exploration of doctors’ communication skills may throw up interesting points about communication in nursing; cancer patients may identify with some of the experiences, thoughts and feelings presented in a study of AIDS patients. In qualitative studies, sampling is often described as purposive or judgement sampling. This means that sampling has been conducted with a particular purpose in mind or on the basis of the researchers’ judgement. In other words, both terms mean that researchers choose a sample that should enable a thorough exploration of issues relevant to the aims of the study. Sampling decisions in qualitative research include the selection of a setting or settings that will allow exploration of the relevant issues. For example, researchers might be interested in how various aspects of policy, practice and staff relationships affect the recovery of patients who have undergone surgery for cancer. They may decide to study just one hospital ward or two or more wards at different hospitals, which can then be compared and contrasted in respect of the issues under study. Once a setting or settings have been chosen, there are decisions to make about who to interview and who or what to observe within the setting(s). This may involve decisions about, for example, which grades of nursing staff to interview, which doctors to interview, whether patients with particular types of cancer might have different things to say, whether to observe all ward rounds, or whether to observe staff in conversation during meal breaks. “Theoretical” sampling is a specialized form of purposive/judgement sampling adopted by researchers who take a grounded theory approach to their work. In theoretical sampling, once a setting has been chosen, researchers make most of their sampling decisions as the research page 24 Research awareness in nursing and midwifery: a workbook progresses. This allows them to select interviewees, for example, who may facilitate exploration of theoretical concerns identified in the data already collected and analysed. Sampling can cease when no new concepts are being generated from the data or, to put it in grounded theory terms, when theoretical categories are “saturated”. This is one way in which sampling, data collection and analysis are linked in grounded theory. When writing research documents, qualitative researchers should make the thinking (including any theoretical influences) behind their sampling decisions explicit. Exercise 9 How are sampling procedures described by King & Jensen? Do you think there are any flaws in the
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