Exam 3 CH6 CH8 CH6 Sampling Theory and Methods Sampling o Involves selecting a relatively small number of elements from a larger defined group of elements o Expects that the information gathered from the small group will enable accurate judgments about the larger group o Used when it is impossibly unreasonable to conduct a census Census o Primary data collected from very member in target population o More time consuming costly than sampling Ex US Census conducted every 10 years Population o Identifiable group of elements of interest to the researcher problem e g people products organizations Ex Mazda Motor Corporation could hire J D Power to measure costumer satisfaction among automobile owners The population of interest could be all people who own automobiles o This would be a problem since there is a lack of specificity and the data collected would not be applicable to customer satisfaction w Mazda Defined Target Population o Consists of complete group of elements people or objects that are identified for investigation based on the objectives of the research project Sampling Units o Target population elements actually available to be used during the sampling process Ex New Mazda automobile purchasers Sampling Frame o List of all eligible sampling units during the sampling process Ex Lists of registered voters and customer lists from magazine publishers or credit card companies Central Limit Theorem CLT o Describes the theoretical characteristics of a sample population o States that for almost all defined target populations the sampling distribution of the mean x or the percentage value p derived from a simple random sample will be approximately normally distributed provided the sample size is sufficiently large when n is or 30 o In other words there is a high probability that the mean of any sample taken from the target population will be a close approximation of the true target population mean as one increases the size of the sample Sampling Error o Any bias that results from mistakes in either the selection process for prospective sampling units or in determining the sample size o Tends to occur because of chance variations in the selection of sampling units o When there is a discrepancy between the statistic estimated from the sample and actual value from the population a sampling error has occurred o Examples Population specific occurs when the researcher does not understand who should be surveyed Survey about breakfast cereal consumption Sampling frame wrong sub population is used to select a sample 1936 presidential election between Roosevelt and Landon Selection occurs when respondents self select their participation in the study Only those that are interested respond Non response occurs when respondents are different than those who do not respond Non sampling Error o Occurs regardless of whether a sample or census is used Ex The target population may be inaccurately defined causing population frame error innapropriate question scale measurements can result in measurement error a questionnaire may be poorly designed causing response error etc o More Examples The wording of questions the order in which they are asked and the number and type of options offered can influence survey results Answers given by respondents do not always reflect their true beliefs because they may feel under social pressure not to give an unpopular or socially undesirable answer Answers given by respondents may be influenced by the desire to impress an interviewer Probability Sampling o Each sampling unit in the defined target population has a known probability of being selected for the sample Nonprobability Sampling unit is not known o Sampling designs in which the probability of selection of each sampling o Selection of sampling units is based on the judgment of the researcher May or may not be representative of target population Simple Random Sampling o A probability sampling procedure in which every sampling unit has a known and equal chance of being selected o Works best for small populations where accurate lists are available Easily understood and can be generalized to defined Advantages target population Disadvantages Difficult to obtain a complete and accurate listing of the target population elements Systematic Random Sampling o Similar to simple random sampling but the defined target population is ordered in some way Usually in the form of a customer list taxpayer roll or membership roster and selected systematically o Less costly quicker than simple random sampling No need to give sampling units any special code prior to drawing the sample Instead sampling units are selected according to their position using the skip interval o Skip Interval Defined target population list size Desired sample size Stratified random sampling o Separation of the target population into different groups called strata and the selection of samples from each stratum o Three Steps 1 Divide target population in homogenous subgroups strata 2 Draw random samples from each stratum 3 Combine the samples from each stratum into a single sample of the target population Proportionately stratified sampling o A stratified sampling method in which each stratum is dependent on its size relative to the population o Larger strata are sampled more heavily because they make up a larger percentage of the target population Disproportionately stratified sampling o A stratified sampling method in which the size of each stratum is independent of its relative size in the population o
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