Sampling - PSY 3213
Sampling - PSY 3213 PSY 3213
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This 9 page Class Notes was uploaded by Emily Notetaker on Monday March 7, 2016. The Class Notes belongs to PSY 3213 at University of South Florida taught by Dr. Brannick in Summer 2015. Since its upload, it has received 21 views. For similar materials see Research Methods in Psychology in Psychlogy at University of South Florida.
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Date Created: 03/07/16
Sampling Obtaining Research Participants Most psychology research questions are pertinent to broad groups of people But, can’t study every person in the world Sampling The process of selecting a number of individuals for a study in such a way that the individuals represent the larger group from which they were selected The larger group from which we SAMPLE people (invite them to participate in study) Population The people invited to participate in study Something about them will be REPRESENTATIVE OF larger Sample group from which they were selected Purpose of Sampling Population Inference Sample Regarding the Sample…. Population (N) If sample is Representative… Sample (n) Regarding the inference… Population (N) Inference THEN INFERENCE IS Sample (n) GENERALIZABLE Mistakes to be conscious of… 1. Sampling bias 2. Sampling error …..which threaten to render a study’s findings invalid Sampling Bias Nonrandom differences (generally the fault of the researcher), which cause the sample is over-represent individuals or groups within the population and which lead to invalid findings o What kind of things cause biased samples? Sampling only those who are easy to contact Might there be systematic differences between those who are easy to contact ad those who are not? Stay-at-home moms are easier to contacts and research than working moms; o But working and stay at home moms might be very different Sampling only those who you CAN contact Might there by systematic diffs. b/w those who you can reach and those who refuse/you can’t find Drug user follow-up studies: o Those who you can contact and those you cannot may be systematically diff. Sampling only those who invite themselves Ex: we have to revel the general idea of what the experiment is about – say, it/s about self-mutilation Some people will be so upset by the idea they won’t sign up for the experiment But those may be different somehow from others o Approaches to Sampling… Biased Nonrandom (less representative) Sampling Sample 1. Convenience Sampling 2. Self-selected 3. Purposive sampling Easy to find (convenience samples) Self-selected Purposive (e.g., snowball sampling) o Convenience Sampling Including whoever happens to be available at the time Students in your class, people on Fowler Street, friends Subjects selected b/c it is easy to access them No reason tied to purposes of research Advantages: Easy to get Disadvantages: Difficulty in determining how much of the effect results from the cause or from sample selection o Purposive Sampling Subjects selected for a good reason tied to purposes of research (drug users, schizophrenics) Why use this? Hard-to-get populations that cannot be found through screening general population Snowball sampling (type of purposive) Selecting a few individuals who can identify other individuals who can identify still other individuals who might be good participants for a study. Streetwalkers Neurosurgeons Advantages: Can say something about very interesting groups of people Disadvantages Doesn’t generalize to population “at large” o Self-selected sampling Worst-case example – internet studies Response rate is a problem But, SONA studies are also self-selected Random/unbiased: allows a procedure governed by chance to select the sample; controls for sampling bias Random sampling is the only method that can, theoretically, ensure a representative sample (one that has the same characteristics as the population) o Steps in sampling: 1. Define population to be sampled Who do you want to make inferences about? 2. Determine sample size (n) How many are enough to get a picture of what’s going on? 3. Control for bias & error This is when you chose your sampling method 4. Select sample Random (unbiased) sampling o Random (unbiased) The process of selecting a sample that allows every individual in the defined pop. To have an equal and independent chance of being selected for the sample o Simple random sample names drawn out of a hat Random phone calls Advantages Easy to conduct Representative of population Strategy requires minimum knowledge of the population to be sampled Disadvantages Need names of all the population members There is difficulty in reaching all selected in the sample o Stratified Divide pop. Into groups that differ in important ways And do random sample from each group A study on some characteristic or mechanism in the population But you think that race makes a difference Classify people into Caucasian, African American, Asian, etc., in the entire population Do simple random sampling from each race Advantages Might be more representative than simple random sampling Disadvantages Need names of all population members Difficulty in reaching all selected in the sample o Oversampling You want to do stratified sampling, but some of your clusters are really small (e.g., Native American) A study on some characteristic or mechanism in the pop. But you think that race makes a difference But there are very few native American people in the community Do simple random sampling from each race but NA and AA You set a percentage of how many AA and NA people you want in the study (say 10%) And from their group, you take more people – but randomly o Cluster Will lead to same result as random sampling Pop. Is divided into groups, usually demographic or organizational Some of the groups are randomly chosen Whose cluster is sampled o Multi-stage sampling Start with same steps as cluster sampling But then add simple random sampling to each cluster Advantages Efficient Researcher doesn’t need names of all population members Disadvantages Fewer sampling points make it less like that the sample is representative o Systematic sampling Selecting individuals within the defined pop. From a list by taking every Kth name Advantages Sample selection is simple Disadvantages Need names of all pop. Members Difficulty in reaching all selected in the sample
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