Unformatted text preview: Sampling
The basics and the background Sampling Background Sections 21 thru 24 in the book External Validity Terminology Statistical Terms in Sampling What is sampling? "The process of selecting UNITS from a POPULATION so by analysis of SAMPLES an investigator may generalize the results back to the POPULATION." Validity Validity: The extent to which an instrument measures what it is intended to measure External Validity: The extent to which conclusions from an instrument used within a sample can be generalized to the population from which the sample was gathered External Validity The approximate truth of conclusions that involve generalizations The degree to which the conclusions in your study would hold for other persons in other places and at other times Two approaches: Sampling model Proximal Similarity Model The Sampling Model for External Validity The Proximal Similarity Model for External Validity External Validity Threats to external validity: Improving external validity: People Places Times Sampling Theory of proximal similarity Replication Validity and Reliability Validity Does the instrument measure what it is supposed to Reliability The degree to which a measure is consistent The degree to which the measure would give you the same result over and over The Different Groups in the Sampling Model Sampling Terminology Population the group to whom you wish to generalize Theoretical Accessible the listing of the accessible population from which you'll draw your sample Sampling frame Sampling Terminology Sample the group of people you select to be in your study Question: The individuals whom you select and who complete the measures are your sample?
(T)rue (F)alse Sampling Terminology Subsample The group of individuals from the sample whom complete and participate in your study Statistical Terms in Sampling Statistics in Samples and Populations Sometimes want to know what is going on just in your sample Standard Deviation: "How Different" your people are from one another You also assume that there are differences in the population at large Standard Error: "How Different" the people in the whole population are from one another The Sampling Distribution
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5 Sample 0 0 0 5 5 5 0 3.0 3.2 3.4 3.6 3.8 4.0 4.2 4.4 0 3.0 3.2 3.4 3.6 3.8 4.0 4.2 4.4 0 3.0 3.2 3.4 3.6 3.8 4.0 4.2 4.4 Average
15 Average Average ...is the distribution of a statistic across an infinite number of samples. The sampling distribution... 10 5 0 3.0 3.2 3.4 3.6 3.8 4.0 4.2 4.4 The Sampling Distribution How do you get from a sample statistic to an estimate of the population parameter? The distribution of an infinite number of samples of the same size as the sample in your study "Average of the averages" is close to the population parameter Sampling distribution is theoretical Standard error and the 65, 95, 99 percent rule The 65, 95, 99 Percent Rule Estimating the Population Using a Sampling Distribution Put another way Sample statistics Population statistics Standard Deviation = the differences in the people in your study Standard Error = the differences in the people in the total population Standard Error of the Mean= the differences in the means of infinite number of studies one could do taken from the population Sampling Distribution Another way of thinking about it How close the mean of your study are likely to be to the "real" mean in the population Summary
Will always have variability in research Population Sample Standard error Standard Deviation Variance Sampling Distribution Sampling error Standard Error of the Mean ...
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- Spring '07
- Statistics, external validity