March 14 - Random sampling error o Sampling error or bias...

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March 14, 2008 Random sampling error o Sampling error or bias: the difference between the sample value and the true population value: Random: due solely to chance Systematics o Statisticians estimate the probability that a sampling result is representative of the population using sampling distribution Systematic sampling error o Refers to differences between the sample and population due to some problem with the sampling method o Major sources of systematic error include Bad sampling frames Non response Bad sampling frames o Gaps in a sampling frame will generate a sample which is not representative of the population Non response o Refers to elements selected from the sample from which no data are collected E.g. people who hung up in telephone survey o Non response can bias the sample if non responders are systematically different from those who do respond E.g. regular drug users may not respond to survey questions on drug use, yielding a sample of people who don’t use drugs as often as in the overall population Result would be an underestimate of use o Non response rates > 30% indicates serious trouble, ability to generalize to population is seriously compromised. Combat systematic error by… o Careful evaluation and correction of sampling frames o Use of cluster sampling or other “frameless” methods o Minimizing non response through repeated efforts to get data from non
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