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# Chap%207%20Hair - Outline of Sampling Lecture A sample or a...

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Outline of Sampling Lecture A sample or a census? Another look at the concept of sampling error Stages in developing a (probability) sampling plan define target population (review) select sampling frame/method Practical considerations when such a frame does not exists, e.g., dog owners in the US select a sampling method what distinguishes probability and nonprobability samples (definitions)
Outline of Sampling Lecture Probability samples (allow for the calculation of sampling error- extended discussion of this, including Milan Sampling Exercise (for SRS of size 25, 100) Confidence intervals in context of SRS (with large, small sample sizes) Discussion of types of probability samples » Simple random sample » Systematic » Stratified (proportionate, disproportionate) » Cluster Discussion on nonprobability samples » convenience » judgment » snowball » quota

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Outline of Sampling Lecture Select Sample Size what do we need to know (i.e., “estimate”) in order to “estimate” sample size? rules of thumb relationship of sample size, confidence, precision, and population variance Boulder Snowfall exercise
Census

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Sampling Sampling is the process of selecting a small number of elements from a larger defined target group of elements such that the information gathered from the small group will allow judgments to be made about the larger groups
The One and Only Goal in Sampling!! Select a sample that is as representative as possible.

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When Is Census Appropriate? Population size itself is quite small Information is needed from every individual in the population, e.g., a vote. Cost of making an incorrect decision is high Sampling errors are high (i.e., very large variance in population), i.e., people sampled are less representative of those not sampled
When Is Sample Appropriate? Sufficient sample size is feasible Both cost and time associated with obtaining information from the entire population is high Quick decision is needed Population being dealt with is relatively homogeneous -- people sampled can better represent those not sampled

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Error in Sampling Total Error Difference between the true population value and the observed value in the sample Sampling Error Error is due to sampling process itself, only occurs in samples, not present in a census. Non-sampling Error Error is observed in both census and sample Examples » Measurement Error » Data Recording Error » Non-response Error
Sampling Error as stated in text: WRONG! Sampling error is any type of bias that is attributable to mistakes in either drawing a sample or determining the sample size

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S/√n analogy: smaller ratio is a more powerful magnifier, more precision Top Row: (left) a photo representing a “census of dots,” (right) a sample of 2000 dots (where dots = n) Bottom Row: (left and right) are samples 1000 and 250 dots, respectively
Nonsampling Error Nonsampling error

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Chap%207%20Hair - Outline of Sampling Lecture A sample or a...

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