Most textbooks on sampling describe this procedure (Levy & Lemsbow, 1980; Thompson, 2002; Yates, 1981). Sampling theory was developed to determine mathematically the most effective way to acquire a sample that would accurately reflect the population under study. Acceptancerate=160(numberaccepting)200(numbermeetingsamplingcriteria)=0.8100%=80%, Acceptancerate=100%refusalrateor100%20%=80%. If the sampling frame is small, the researcher can write names on slips of paper, place the names in a container, mix well, and draw out one at a time until the desired sample size has been reached. Using random sampling, the researcher cannot decide that person X would be a better subject for the study than person Y. Confused about theoretical sampling? See Table 17-10 for examples of probability sampling from the literature. Am J Nurs. Sampling involves selecting a group of people, events, behaviors, or other elements with which to conduct a study. Because of the importance of generalizing, there are risks to defining the accessible population too narrowly. Key concepts of sampling theory include populations, target population, sampling or eligibility criteria, accessible population, elements, representativeness, sampling frames, and sampling methods or plans. The higher the retention rate, the more representative the sample is of the target population, and the more likely the study results are an accurate reflection of reality. 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. Sampling theory was developed to determine the most effective way to acquire a sample that accurately reflects the population under study. Fouladbakhsh and Stommel (2010, p. E8) used multistage cluster sampling in their study of the complex relationships among gender, physical and psychological symptoms, and use of specific CAM [complementary and alternative medicine] health practices among individuals living in the United States who have been diagnosed with cancer. These researchers described their sampling method in the following excerpt from their study. Validity and reliability are two statistical properties used to evaluate the quality of research instruments (Anastasi, 1986). Sampling theory in medical science is a study of gathering information about a population by investigating only a part of it. Unauthorized use of these marks is strictly prohibited. Thus, persons who are legally or mentally incompetent, terminally ill, or confined to an institution are more difficult to access as subjects (see Chapter 9). Share this:Click to share on Twitter (Opens in new window)Click to share on Facebook (Opens in new window)Click to share on Google+ (Opens in new window) The sampling criteria determine the target population, and the sample is selected from the accessible population within the target population (see Figure 15-1). ere is a single objective underlying sampling theory: to create a subset of a population in which all cases in the subset have an equal probability of being selected from the population. 18. In the first stage, 339 primary sampling units were selected from about 1,900 area sampling units representing counties, groups of adjacent counties, or metropolitan areas covering the 50 states and the District of Columbia [1st stage cluster sampling]. Sampling criteria may include characteristics such as the ability to read, to write responses on the data collection instruments or forms, and to comprehend and communicate using the English language. Data is temporarily unavailable. Quantitative, outcomes, and intervention research All samples with human subjects must be volunteer samples, which includes individuals willing to participate in the study, to protect the rights of the individuals (Fawcett & Garity, 2009). PMC 62 The remaining 26 women (exercise = 14; comparison = 12) withdrew from the study before 24 months. As the name suggests, nonprobability sampling does not use random sampling techniques in creating the study sample. In some cases, this random selection continues through several stages and is referred to as multistage cluster sampling. Probability samples reduce sampling error. 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). If the first name is not replaced, the remaining 49 names have a 9 in 49 chance, or a 0.18 probability, of being selected. However, even in a random sample, systematic variation can occur if potential subjects decline participation. 2010 Sep 1;12(3):369-74. doi: 10.1111/j.1442-2018.2010.00541.x. sampling when appraising research, and discusses sampling characteristics we should look for in quantitative and qualita tive studies. Systematic variation is greatest when a high number of subjects withdraw from the study before the data have been collected or when a large number of subjects withdraw from one group but not the other in the study (Kerlinger & Lee, 2000; Thompson, 2002). Simple random sampling is the most basic of the probability sampling methods. 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. When the study is complete, the findings are generalized from the sample to the accessible population and then to the target population if the study has a representative sample (see the next section). Copyright 2021 Wolters Kluwer Health, Inc. All rights reserved. Because of space restrictions, this editorial focuses on the randomised controlled trial (RCT) as an example of quantitative research,and grounded theory as an exampleofqualitativeresearch . This correlation can cause a decrease in precision and an increase in sampling error. The values are randomly scattered around the mean. Tags: The Practice of Nursing Research Appraisal Synthesis and Gener Critical questions are provided to help researchers choose a sampling method. 4. There are many types of sampling methods, but most sampling falls into two main categories: probability sampling, and non-probability sampling. "Sampling theory was developed to determine the most effective was of acquiring a sample that accurately reflects the population under study. Wolters Kluwer Health, Inc. and/or its subsidiaries. You may hold opinions about the adequacy of these techniques, but there is not enough information to make a judgment. Stratification ensures that all levels of the identified variable, in this example age, are adequately represented in the sample. 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%). Again, these units could be people, events, or other subjects of interest. The sample of 75 women was randomly selected, but the researchers did not indicate the process for the random selection. For each person in the target or accessible population to have an opportunity to be selected for the sample, each person in the population must be identified. Sampling error limits generalizability and research accuracy (validity) by introducing bias into the study. Sampleretentionrateformula=numbersubjectscompletingstudysamplesize100% 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 term "sampling theory" is simply a fancy way to say the practice of asking people to take a test for you. For example, one could conduct a study in which the defined population was all living recipients of heart and lung transplants. In descriptive or correlational studies, the sampling criteria may be defined to ensure a heterogeneous population with a broad range of values for the variables being studied. The first situation is when a simple random sample would be prohibitive in terms of travel time and cost. If potential subjects have diminished autonomy or are unable to give informed consent, consent must be obtained from their legal representatives. The nurse researcher may further decide to require specific proportions of admission diagnoses in the sample. A, You might identify broad sampling criteria for a study, such as all adults older than 18 years of age able to read and write English. There are five main types of probability sampling including simple random sampling, systematic sampling, stratified sampling, cluster sampling, and multi-stage sampling. The values are randomly scattered around the mean. For example, the researcher places a pencil on 58 in. Often researchers identify either the attrition rate or the retention rate but not both. Figure 15-1 Population, sample, and subject selected for a study. Chapter 15 sampling method was utilized, wherein participants introduced other . Ebling Library, Health Sciences Learning Center 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. We can estimate the population parameter by identifying the values obtained in previous studies examining the same variables. There can be some differences in the probability for the selection of each element, depending on whether the name or number of the selected element is replaced before the next name or number is selected. Sample Attrition and Retention Rates in Studies This chapter examines sampling theory and concepts; sampling plans; probability and nonprobability sampling methods for quantitative, qualitative, outcomes, and intervention research; sample size; and settings for conducting studies. Unauthorized use of these marks is strictly prohibited. A Quick Primer on Sampling Theory In systematic random sampling (SYS), units are selected from the frame at regular intervals, and a sampling interval and a random start are required. Figure 15-1 shows the relationships among the population, target population, and accessible populations. Keyword Highlighting Theoretical sampling Twiss et al. The list for PAs was derived from the American Academy of Physicians Assistants (AAPA), and a comprehensive list of NPs was derived from the medical and nursing boards of the 50 states and the District of Columbia [sampling frames for NPs and PAs]. After undeliverable (1.9%) and other disqualified respondents (13.2%, i.e., no longer practicing, non-primary-care practitioner) were removed, the overall adjusted response rate was 50.6%. (Ulrich et al., 2006, p. 393). The target population is the entire set of individuals or elements who meet the sampling criteria, such as women who have experienced a myocardial infarction in the past year. These researchers obtained their sample using a simple random sampling method that is described in the following excerpt from their study. Attritionrate=40(numberwithdrawing)160(samplesize)=0.25100%=25%. The sample is obtained from the accessible population by a particular sampling method, such as simple random sampling. A sampling method is the process of selecting a group of people, events, behaviors, or other elements that represent the population being studied. 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. This study included clearly identified inclusion and exclusion sampling or eligibility criteria that are presented in the following excerpt. If the accessible population is limited to a particular setting or type of setting, the individuals seeking care at that setting may be different from the individuals who would seek care for the same problem in other settings or from individuals who self-manage their problems. Federal government websites often end in .gov or .mil. The likelihood is increased that the sample is representative of the target population and the results are an accurate reflection of reality. 94 Twiss et al. 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. Biases may be introduced that make generalization to the broader target population difficult to defend. A sampling method is similar to a design; it is not specific to a study. Epidemiologists sometimes use entire populations for their large database studies. Thus, persons who are legally or mentally incompetent, terminally ill, or confined to an institution are more difficult to access as subjects (see Chapter 9). Cluster sampling provides a means for obtaining a larger sample at a lower cost. For example, if your study examines attitudes toward acquired immunodeficiency syndrome (AIDS), the sample should represent the distribution of attitudes toward AIDS that exists in the specified population. Imagine trying to arrange personal meetings with 100 people, each in a different part of the United States. Purposeful sampling for qualitative data collection and analysis in mixed method implementation research. Freedman D, et al. Boston: Cengage Learning; 2006. p. 76396. If the sampling frame is small, the researcher can write names on slips of paper, place the names in a container, mix well, and draw out one at a time until the desired sample size has been reached. Probability samples reduce sampling error. 72 Random sampling leaves the selection to chance and decreases sampling error and increases the validity of the study (Thompson, 2002). The next column will discuss measurement in quantitative research, including the concepts of reliability and validity. Each member of the sampling frame has only one chance of being chosen for inclusion in the study sample.4 For example, suppose a nurse researcher is interested in workplace satisfaction among the 200 medicalsurgical nurses within a health care system. Unable to load your collection due to an error, Unable to load your delegates due to an error. 11. Common methods of nonprobability sampling include the following2: In convenience sampling, a researcher's selection of participants is chiefly driven by the availability of potential research participants.2, 4 For example, a convenience sample might comprise all patients admitted to a specific clinical unit during the study period. Potential subjects cannot be excluded just because they are too sick, not sick enough, coping too well, or not coping adequately. In a second step, primary sampling units were partitioned into substrata (up to 21) based on concentrations of African American and Hispanic populations [2nd stage cluster sampling]. Please read ahead to understand more about what this program does. From the population we take a sampling to create the sample size. The sample must be randomly selected from the sampling frame. 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%). Sampleretentionrateformula=numbersubjectscompletingstudysamplesize100%, Retentionrate=120(numberretained)160(samplesize)=0.75100%=75%. National Library of Medicine In: Statistics. States, cities, institutions, or organizations are selected randomly as units from which to obtain elements for the sample. Sampling theory is the study of relationships existing between a population and samples drawn from the population. An Introduction to Sampling Theory The applet that comes with this WWW page is an interactive demonstration that will show the basics of sampling theory. All samples with human subjects must be, For each person in the target or accessible population to have an opportunity to be selected for the sample, each person in the population must be identified. Thus, probability sampling methods are often referred to as random sampling methods. The sampling plan must be described in detail for purposes of critical appraisal, replication, and future meta-analyses. Exclusion criteria tend to increase the systematic bias in the sample and increase the sampling error. Quantitative researchers opt for nonprobability sampling techniques largely for convenience and containment of cost and time. Bethesda, MD 20894, Web Policies Am J Nurs. Ulrich et al. Table 15-2 is useful only if the population number is less than 100.

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