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Marketing and Consumer Studies
MCS 3030

Chapter 2: Sampling Population- group you want to generalize to and the group you sample from in a study Theoretical Population- former population – who you want to generalize to Accessible Population- latter population – population you can get access to Sample-actual units you select to participate in your study Sampling- process of selecting units (ie. People, organizations) from a population of interest so that studying a sample you can generalize results to population from which units were chosen Sampling frame-list from which you draw sample (sometimes no list and samples drawn from explicit rule (ie. When population is people who walk by then sampling frame is population of people who pass by in time frame and rules you use to decide whom to select) External Validity External validity- degree to which conclusions in study would hold for other persons in other places and at other times -related to generalizing a sample to population -refers to approximate truth of conclusions that involve generalization generalizability- degree to which study conclusions are valid for members of population no included in study sample -two approaches to provide evidence for generalization: 1.Sampling model- model for generalizing in which you identify your population, draw a fair sample, conduct your research and finally generalize results to other population groups (Population-Draw Sample-Generalize back to Population) Problems: -at time of study you may not know what part of population you will want to generalize to -may not be able to draw a fair or representative sample easily -impossible to sample across all times that you may like to generalize to (like next year) 2. Proximal similarity model –model for generalizing from your study to another context based upon the degree to which the other context is similar to your study -think about different generalizability contexts and develop theory about which contexts are like your study and which are less so (place different contexts in terms of relative similarities and then can generalize results of study to other people, places or times) Proximal- means nearby Gradient of similarity- dimension along which your study context can be related to other potential contexts to which you might wish to generalize. Contexts that are closer to yours along the gradient of similarity of place, time, people, and so on can be generalized to with more confidence than ones that are further away. Threats to External Validity -explanation of how you may be wrong in making generalization (people, places, times) Improving External Validity Based on sampling model: -do a good job drawing sample from population -use random sample rather than nonrandom procedure -random selection-process or procedure that assures that the different units in your population are selected by chance -once selected, try to ensure respondents participate in study (keep dropout rates low) Based on proximal similarity model: -could do better job describing ways contexts differ from others (provide data about similarity between various groups of people, places, and times) -map degree of proximal similarity among various contexts with concept mapping Concept mapping-two dimensional graphs of a groups ideas where ideas that are more similar are located closer together and those judged less similar are more distant (often used by group to develop conceptual framework for research project) -show critics they’re wrong (do study in variety of places, with different people at different times) Sampling Terminology Census- kind of survey that involves a complete enumeration of the entire population of interest Statistical Terms in Sampling Response- specific measurement value that a sampling unit supplies Statistic- process of estimating various features from data, often using probability theory -used when you look across the responses for your entire sample Population parameter- mean (average) you would obtain if you were able to sample entire population The Sampling Distribution Sampling distribution- theoretical distribution of an infinite number of samples of the population of interest in your study Bell curve- smoothed histogram (bar graph) describing expected frequency for each value of a variable Standard deviation (SD) - spread of variability of the scores around their average in a single sample -square root of variance Standard error- spread of averages around the average of averages in a sampling distribution Sampling Error -in sampling, standard error is called sampling error Sampling error- error in measurement associated with sampling -gives you some idea of the precision of your statistical estimate -low sampling error means that you had relatively less variability or range in sampling distribution -calculate sampling error by basing calculation on standard deviation of your sample -greater sample’s standard deviation, greater standard error (and sampling error) -greater sample size the smaller the standard error (because greater sample size the closer your sample is to actual population itself) -if take sample that consists of entire population than there is no sampling error because you don’t have a sample, you have the entire population (consensus) and therefore the mean you estimate is parameter -estimate standard error (sampling error) based on standard deviation Systematic error- random errors in measurements are caused by unknown and unpredictable changes in the study (changes may occur in measuring instruments or environmental conditions) Systematic error- systematic errors in measurements usually come from the instrument or the person conducting the study (may occur because there’s something wrong with instrument or in data handling system or because instrument is wrongly used by the experimenter) -types: sample frame, non response The 68, 95, 99 Percent Rule (Empirical Rule) -applies to standard deviation and standard error -symmetrical distributions =bell curve=normal distribution -one standard unit -68% of cases in distribution -two standard units- 95% of cases in distribution -three standard units
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