Since researchers generally do not have access to the full population of interest for a research project (the target population), they must rely on studying a subset of that population (the study sample or sample population). A sampling plan defines the process of making the sample selections; sample denotes the selected group of people or elements included in a study. Search for Similar Articles https://researchguides.library.wisc.edu/nursing, Types of Research within Qualitative and Quantitative, Independent Variable VS Dependent Variable, Nursing Scope & Standards of Practice, 4th Ed, Find Instruments, Measurements, and Tools. In a third step, clusters of dwelling units form the secondary sampling units selected from each substratum [3rd stage cluster sampling]. Nursing Research, Step by Step is coordinated by Bernadette Capili, PhD, NP-C: [emailprotected] The authors have disclosed no potential conflicts of interest, financial or otherwise. (2009) identified that 249 participants or subjects met the sampling criteria and 249 were enrolled in the study indicating that the acceptance rate for the study was 100%. Systematic Variation Methods of achieving random sampling are described later in the chapter. Network sampling clearly violates both assumptions of probability samplingrandom and independent selectionand therefore is a nonprobability sampling method intended to develop a deeper theoretical understanding and does not allow for generalizability. 28 Essentials of epidemiology in public health. The sample retention was 223 women for a retention rate of 90% (223 249 100% = 89.6% = 90%), and the sample attrition rate was 26 women for an attrition rate of 10% (100% 90% = 10%). Systematic variation or bias is most likely to occur when the sampling process is not random. A study might have inclusion or exclusion sampling criteria (or both). What Is Sampling Theory in Nursing Research? Using random sampling, the researcher cannot decide that person. Hence, sampling theory also consists of estimation methods. Section from a Random Numbers Table The only nursing research and theory book with primary works by the original theorists. National Center for Biotechnology Information, Lister Hill National Center for Biomedical Communications, Agency for Healthcare Research and Quality, Centers for Disease Control and Prevention, Robert Wood Johnson Foundation County Health Rankings & Roadmaps, Centers for Medicare and Medicaid Services. Because this is a quasi-experimental study that examined the impact of the treatment on the dependent or outcome variables, the increased controls imposed by the sampling criteria strengthened the likelihood that the study outcomes were caused by the treatment and not by extraneous variables. The treatment group retention was 110 women with a retention rate of 89% (110 124 100% = 88.7% = 89%). Most textbooks on sampling describe this procedure (Levy & Lemsbow, 1980; Thompson, 2002; Yates, 1981). The individual units of the population and sample are called elements. For example, individuals who successfully lose weight would be a hypothetical population. Stratification is not as useful if one stratum contains only a small number of subjects. The likelihood is increased that the sample is representative of the target population and the results are an accurate reflection of reality. The term used by researchers depends of the philosophical paradigm that is reflected in the study and the design. Randomization Boston: Cengage Learning; 2006. p. 76396. Nonprobability sampling. This listing of members of the population is referred to as the sampling frame. Using theoretical sampling, the qualitative researcher intentionally samples additional participants to gain further insight into information obtained from previous study participants to inform ongoing theory development. In stratified random sampling, the subjects are randomly selected on the basis of their classification into the selected strata. However, the study would have been strengthened by a discussion of the process for random sampling and a clarification of how the subjects were assigned to groups. Age limitations are often specified, such as adults 18 years and older. Simple random sampling is the most basic of the probability sampling methods. From a sampling theory point of view, randomization means that each individual in the population should have a greater than zero opportunity to be selected for the sample. Sampling theory is the study of relationships existing between a population and samples drawn from the population. The remaining 26 women (exercise = 14; comparison = 12) withdrew from the study before 24 months. 33 The chapter concludes with a discussion of the process for recruiting and retaining subjects or participants for study samples in various settings. With a stratified random sample, you could use a smaller sample size to achieve the same degree of representativeness as a large sample acquired through simple random sampling. This situation could occur, for example, if all the subjects were college students, which has been the case in the development of many measurement methods in psychology. To achieve these goals, researchers need to understand the techniques of sampling and the reasoning behind them. 11. Often researchers identify either the attrition rate or the retention rate but not both. Editor's note: This is the third article in a series on clinical research by nurses. 32 National Library of Medicine Usually researchers report either the acceptance rate or the refusal rate but not both. Purpose or purposeful sampling The use of a computer to select a sample randomly is usually the most efficient and unbiased process. Epidemiologists sometimes use entire populations for their large database studies. In these types of studies, the sampling criteria need to be specific and designed to make the population as homogeneous or similar as possible to control for the extraneous variables. The aim of sampling is to approximate a larger population on characteristics relevant to the research question, to be representative so that researchers can make inferences about the larger population. government site. Please try again soon. An official website of the United States government. For example, if study participants who choose to leave the study tend to be poorer with less education, the nurse researcher will have difficulty generalizing the study findings to these populations. Sampling Design in Nursing Research Editor's note: This is the third article in a series on clinical research by nurses. The accessible population must be representative of the target population. Purposeful sampling, also known as selective sampling, is used in both quantitative and qualitative research. The sampling theory was established to help find the most suitable method of acquiring a sample that exactly represents the study population using mathematical formulae. A self-administered questionnaire was mailed to an initial stratified random sample [sampling method] of 3,900 NPs and PAs practicing in the United States. Explore the historical and contemporary theories that are the foundation of nursing practice today. If the accessible population is defined as individuals in a white, upper-middle-class setting, one cannot generalize to nonwhite or lower income populations. The following excerpt from this study describes the sampling method used to obtain the final sample of 1536 providers (833 NPs and 689 PAs). Random sampling can be: simple random sampling stratified random sampling, and There are two primary types of sampling methods that you can use in your research: Probability sampling involves random selection, allowing you to make strong statistical inferences about the whole group. HHS Vulnerability Disclosure, Help 8th ed. While more cost-effective and often more convenient, nonprobability sampling increases the risk of sampling bias and therefore limits generalizability and creates threats to research validity. An extreme example of this problem is the highly restrictive sampling criteria used in some experimental studies that result in a large sampling error and greatly diminished representativeness. The sample selection strategy (the sampling plan) is constructed with the intent to minimize selection bias in the selection of participants from within the sampling frame. In addition, a sample must represent the demographic characteristics, such as age, gender, ethnicity, income, and education, which often influence study variables. Sample Attrition and Retention Rates in Studies The treatment group retention was 110 women with a retention rate of 89% (110 124 100% = 88.7% = 89%). The retention rates for both groups were very strong and comparable (treatment group 89% and comparison group 90%). Probability samples reduce sampling error. Probability and Nonprobability Sampling Methods Commonly Applied in Nursing Research. You may hold opinions about the adequacy of these techniques, but there is not enough information to make a judgment. The nurse researcher may further decide to require specific proportions of admission diagnoses in the sample. However, in quasi-experimental or experimental studies, the primary purpose of sampling criteria is to limit the effect of extraneous variables on the particular interaction between the independent and dependent variables. Twiss et al. In: 5. While the purpose of stratified random sampling is to improve participant representation, the purpose of cluster sampling is to improve sampling efficiency, thus reducing time and cost.3. Similar to stratified random sampling, cluster random sampling uses natural geographic and organizational clusters of potential research participants to create a sampling frame.2 For example, in exploring work satisfaction among clinical nurses using a cluster sampling methodology, the researcher would randomly select cities from which to draw the sample, then randomly select hospitals from within each city, and finally randomly assign nurses from within each hospital to participate in the study. Theoretical sampling is a qualitative sampling technique that evolves over the course of a study as the researcher begins to understand more from the emerging data. However, the sample was a great strength of this study and appeared to represent the target population of NPs and PAs currently practicing in primary care in the United States. High refusal rates to participate in a study have been linked to individuals with serious physical and emotional illnesses, low socioeconomic status, and weak social networks (Neumark, Stommel, Given, & Given, 2001). The sample must be randomly selected from the sampling frame. Wooldridge JM. In the aforementioned situation, if proportions are used and the sample size is 100, the study would include only five Asians, hardly enough to be representative. A sample of 249 participants met the screening criteria and they were enrolled in the study. Of the 249 women, 223 completed the 24-month testing and were included in the analysis (exercise [treatment group] = 110; comparison = 113). Systematic bias increases as the subjects refusal rate increases. To avoid disparities in the representation of any one hospital in a random sample of clinical nurses within the health care system, the researcher can use stratified random sampling to randomly select a designated number of nurses within each hospital. Another technique is to assign a number to each name in the sampling frame. The 833 NPs and 689 PAs add to 1522 subjects and it is unclear why the sample size is identified as 1536 unless there are missing data from subjects. Precision in estimating parameters requires well-developed methods of measurement that are used repeatedly in several studies. However, this study may systematically underrepresent employed adults who were unable to answer the phone during work hours, therefore limiting the generalizability of the study. PMC Degirmen et al. Highlight selected keywords in the article text. 15 Rodriguez J, et al. If your sample is very similar to the population you have a strong case to say that the same things you found in the sample also apply in the population. However, random sampling must take place in an accessible population that is representative of the target population. Stratified random sampling is used when the researcher knows some of the variables in the population that are critical to achieving representativeness. Sampling methodology influences how well the study represents the target population, and thereby affects the generalizability of the study. Random sampling increases the extent to which the sample is representative of the target population. Quota sampling is a variant of convenience sampling that controls for some characteristics within the target population.2 Relative to a convenience sample, a quota sample may better represent certain subpopulations of interest to the researcher. The NHIS [National Health Interview Survey] methodology employs a multistage probability cluster sampling design [sampling method] that is representative of the NHIS target universe, defined as the civilian noninstitutionalized population (Botman, Moore, Moriarty, & Parsons, 2000, p. 14; National Center for Health Statistics). In some studies, the entire population is the target of the study. Statistical sampling theory provides a powerful theoretical framework for generalizing from samples to corresponding populations and is most relevant when generalizing to populations of units and settings (external validity question 1) that can be enumerated and are under the control of the researchers. E8-E9) With a stratified random sample, you could use a smaller sample size to achieve the same degree of representativeness as a large sample acquired through simple random sampling. Imagine trying to arrange personal meetings with 100 people, each in a different part of the United States. Within the hospitals, nursing units might be randomly selected. You may be trying to access this site from a secured browser on the server. Non-probability sampling methods are those in which elements are chosen through non-random methods for inclusion into the research study and include convenience sampling, purposive sampling, and snowball sampling. 07 16. Using random sampling, the researcher cannot decide that person X would be a better subject for the study than person Y. States, cities, institutions, or organizations are selected randomly as units from which to obtain elements for the sample. Sampling error occurs as a result of random variation and systematic variation. The sampling methods to be included in this text are identified in Table 15-1 and are linked to the types of research that most commonly incorporate them. These studies are referred to as population studies (Barhyte, Redman, & Neill, 1990). These sampling criteria probably were narrowly defined by the researchers to promote the selection of a homogeneous sample of postmenopausal BCSs with bone loss. Cluster sampling provides a means for obtaining a larger sample at a lower cost. Freedman D, et al. The total 75 study patients [sample] out of the 281 were selected by random sampling method from the patients presenting orders [sampling frame] and evenly divided into three groups; a control group, a foot and hand massage group, and a foot massage group, each of which included 25 patients. Because some patients accepted the intervention before the operation, but changed their mind after the operation (3 patients in total), not all patients participated in the study. (Degirmen et al., 2010, p. 154) TABLE 15-2 Each of these sampling strategies has trade-offs. The https:// ensures that you are connecting to the Quantitative, outcomes, and intervention research Sampling error is the approximate difference between the results from a sample of people from a larger group, and the likely results of studying every single person in that group. These studies are referred to as population studies (Barhyte, Redman, & Neill, 1990). There are also the difficulties of sampling student performance over time and of controlling test conditions (p. 1). Federal government websites often end in .gov or .mil. For a sample to be representative, it must be similar to the target population in as many ways as possible. Qualitative research The term used by researchers depends of the philosophical paradigm that is reflected in the study and the design.