Population, Sample, and Sampling Techniques. Identify the Unit of Analysis. Unit of Analysis 4/9/2013. Dr. K. A. Korb UniJos

Transcription

1 Develop Research Question Plan Research Design Population, Sample, and Sampling Techniques Measurement Sampling Dr. K. A. Korb UniJos Data Collection Data Processing Data Analysis and Interpretation Unit of Analysis Unit of analysis:who or what is being described or analyzed. Identify the Unit of Analysis 1. Examine the relationship between self concept and academic achievement in university students. 2. Identify differences between trained and untrained counselors in their counseling effectiveness. 3. Identify the number of hospitals that have a trained psychologist. 4. Determine the effectiveness of peer counseling on JSS students test anxiety. 5. Compare the Yoruba and Hausa beliefs of the causes of mental health problems. 6. Examine the relationship between community support and PTSD symptoms among youth in Jos. 1

2 Population Target population: Group of people that the researcher wants to draw a conclusion about once the research study is finished. Specify the criteria that determines which cases are included and which cases are not included. APAspecifies the following demographic characteristics of the participants must be explained: Sex Age Race/ethnicity Socioeconomic status (where appropriate) Disability status (where possible and appropriate) Sample Study every individual in the target population would cause the following problems: Cost Difficulty finding the entire population Unqualified research assistants would be necessary, thus reducing the quality of data Time to distribute and collect the questionnaires would take years Sample: Subset of cases selected from a population who actually take part in the study Sampling Sampling procedures have been developed to identify a sample that is representativeof the population Representative Sample: The sample closely resembles the target population on the main relevant characteristics Theory of Sampling Researchers want to gather information about a whole group of people (the population). Researchers only observe a part of the population (the sample). The findings from the sample are generalized,or extended, back to the population. 2

3 Theory of Sampling Sampling Steps 1. Identify the unit of analysis. Sampling technique selects cases based on this unit of analysis. 2. Determine how many units need to be sampled. Advantages of larger samples Generally more representative of the target population Provide stronger statistical power Disadvantage of larger samples Decrease the quality of the research study, particularly for experimental and quasi-experimental designs. Sample Sizes Descriptive designs require at least 100 participants Correlational designs require at least 50 participants Experimental, quasi-experimental, and causalcomparative designs require at least 15 participants per group. Sampling Steps 3. Select a Sampling Technique 4. Implement the Sampling Technique following the key principles of the Sampling Technique 3

4 Types of Sampling Designs Probability Sampling: All cases in the population are randomly selected and have a known probability of being included in the sample Nonprobability Sampling: The chances of selecting any case are not known because cases are nonrandomly selected Advantage of Probability Sampling: Investigator bias will not affect selection of cases Disadvantage of Probability Sampling: Not always possible or cost-effective Simple Random Sampling Simple Random Sampling: Every individual in the target population has an equal chance of being part of the sample. Steps in Simple Random Sampling: 1. Obtain a complete list of the population 2. Randomly select those cases for the sample via hat and draw or random number table Revision from Research Designs Simple Random Sampling Random:Technical term that means that every case has an equal and independent chance of being assigned or selected Selection:How participants were selected from the population to participate as the sample of the study. Assignment:How participants are assigned to the treatment and control groups RANDOM cannot be used haphazardly, but can only be used to describe specific procedures that ensure that every case had an equal and independent chance of being selected. 4

5 Stratified Sampling Stratified random sampling: Divide the population into groups based on a relevant characteristic and then selects participants within those groups. The purpose is to ensure that subgroups are adequately represented in the sample. Stratified Sampling Steps in stratified random sampling 1. Determine the stratathat the population will be divided into. Strata: Relevant characteristic that needs representation in the sample 2. Determine the number of participants necessary for each strata Equal representation within the strata OR Proportionate sampling:the proportion in the sample matches the proportion in the population. 3. Split the units of analysis into the respective strata. 4. Randomly sample participants from within each strata Stratified Random Sampling Systematic Sampling Systematic Sampling: Selecting every K th case from a complete list of the population Steps in Systematic Sampling: 1. Obtain a complete list of the population 2. Identify K Possibly divide the size of the population by the desired sample size 3. Start at a RANDOM point in the list of the population 4. Include every K th person on the population list 5

6 Systematic Sampling: K=2; Random Number = 5 Population Multi-Stage Sampling Multi-Stage Sampling: Sample is selected in multiple steps, or stages. If the unit of analysis is not selected in the first step, then the sampling procedure is multi-stage sampling. Various sampling techniques may be used at the different stages An alternate name for multi-stage sampling is Cluster Sampling Multi-Stage Sampling Multi-Stage Sampling Steps in Multi-Stage Sampling: 1. Organize the sampling process into stages where the unit of analysis is systematically grouped. 2. Select a sampling technique for each stage 3. Systematically apply the sampling technique to each stage until the unit of analysis has been selected 6

7 Purposive Sampling Purposive Sampling: Researcher uses their expert judgment to select participants that they judge are representative of the population. Steps in Purposive Sampling 1. Consider factors that might influence the outcomes of the study 2. Purposefully select a sample that adequately represents the target population on these variables Convenience Sampling Convenience Sampling: Select a requisite number of cases that are conveniently available Conclusion The key question in sampling is How representative is the sample of the target population? The researcher has the burden of demonstrating in their report that the sample, regardless of how it was chosen, represents the target population. Probability sampling will typically answer this question the best. However, as long as the researcher makes a convincing argument that the sample adequately represents the target population, the researcher can use any available sampling procedure. 7