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11. Populations and Samples. Learning Objectives. Define Population And Sample Distinguish Between Target And Accessible Population Discuss Probability And Nonprobability Sampling Procedures Compare Four Methods Of Probability Sampling. Learning Objectives.

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## Populations and Samples

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**11**Populations and Samples**Learning Objectives**• Define Population And Sample • Distinguish Between Target And Accessible Population • Discuss Probability And Nonprobability Sampling Procedures • Compare Four Methods Of Probability Sampling**Learning Objectives**Compare Three Methods Of Nonprobability Sampling Determine Which Sampling Technique To Use In Various Types Of Research Studies Compare Longitudinal And Cross-Sectional Studies Enumerate Factors To Be Considered In Deciding The Size Of The Sample**Learning Objectives**Discuss Sampling Error And Sampling Bias Critique The Sampling Procedure Described In Research Reports And Articles**Population**• Complete set of persons or objects • Common characteristic • Of interest to the researcher**Sample**• Subset of a population • Sample represents the population.**Sample Selection**• Representation of the population • Method for getting the sample • Sample size for the study**Sample Terms**• Element • Single member of a population • Sampling frame • Listing of all elements • Study sample, if from this frame**Learning Objective TwoDistinguish Between Target And**Accessible Population**Population Terms**• Target population • Accessible population**Target Population**• Definition • Entire group of people or objects • People or objects meet designated set of criteria. • Generalization of the findings**Accessible Population**• Definition • Group of people or objects • Researcher has access to them.**Population Importance**• Conclusions based on data • Data from accessible population • Decisions made from study results**Learning Objective ThreeDiscuss Probability And**Nonprobability Sampling Procedures**Types of Sampling Methods**• Probability • Nonprobability**Probability Sampling**• Uses random sampling procedures • Selects sample from elements or members of population • Types • Simple • Stratified • Cluster • Systematic**Nonprobability Sampling**• Uses nonrandom sampling procedures • Selects sample from elements or members of population • Types • Convenience • Quota • Purposive**Learning Objective FourCompare Four Methods Of Probability**Sampling**Probability Sampling**• Simple random • Stratified • Cluster • Systematic**Random Selection**• Key word in sample selection • Every subject has an equal chance.**Probability Sampling**• Allows researcher to estimate the chance • Helps with inferential statistics with greater confidence • Gives the ability to generalize the findings**Simple Random Sampling**• Type of probability sampling • Importance of this sampling • Equal chance of selection • Independent chance of selection**Advantages ofSimple Random Sampling**• Little knowledge of population is needed. • Most unbiased of probability method • Easy to analyze data and compute errors**Disadvantages ofSimple Random Sampling**• Complete listing of population is necessary. • It is time consuming to use.**Steps for Simple Random Sampling**• Identify the accessible population or list of elements • Choose the method for getting the sample • Note how easy it is through this example • Names of elements on slips of paper • Papers are placed into a hat. • Individual draws a slip of paper. • Individual continues until sample number is met.**Stratified Random Sampling**• Type of probability sampling • Population is divided into subgroups or strata. • Simple random sample taken from each strata**Approaches forStratified Random Sampling**• Proportional stratified sampling • Determine sampling fraction for each stratum • Ensure that this stratum is equal • Proportion in total population • Disproportional stratified sampling • Determine stratum is represented • Used when strata are very unequal • Note the key word disproportional**Advantages of StratifiedRandom Sampling (cont’d)**• Increases probability of being representative • Ensures adequate number of cases for strata**Disadvantages ofStratified Random Sampling**• Requires accurate knowledge of population • May be costly to prepare stratified lists • Statistics are more complicated.**Cluster Random Stratified Sampling**• Large groups or clusters, not people, are selected from population. • Simple, stratified or systematic random sampling may be used during each phase of sampling.**Advantages ofCluster Random Sampling**• Saves time and money • Arrangements made with small number sampling units • Characteristics of clusters or population can be estimated.**Disadvantages ofCluster Random Sampling**• Causes a larger sampling error • Requires each member assignment of population to cluster • Uses a more complicated statistic analysis**Systematic Random Sampling**• Type of probability sampling • Every kth element is selected. • Process • Obtain a listing of population • Determine the sample size • Determine the sampling interval (k = N/n) • Select random starting point • Select every kth element**Advantages ofSystematic Random Sampling**• Easy to draw sample • Economical • Time-saving technique**Disadvantages ofSystematic Random Sampling**• Samples may be biased. • After first sample is chosen, no longer equal chance**Learning Objective FiveCompare Three Methods Of**Nonprobability Sampling**Nonprobability Sampling**• Sample elements are chosen nonrandomly. • Produces biased sample • Each element of the population may not be included in the sample. • Restricts generalizations made about study findings**Nonprobability Sampling**• Convenience • Quota • Purposive**Convenience Sampling**• Chooses the most readily available subject or object • Does not guarantee that the subject or object is typical of the population**Snowball Sampling**• Type of convenience sampling method • Study subjects recruit other potential subjects. • May be called network sampling • May find people reluctant to volunteer**Quota Sampling**• Type of nonprobability sampling • Researcher selects sample to reflect characteristics. • Examples of stratum**Quota Sampling**• Age • Gender • Educational background • Number of elements in each stratum • Number is in proportion to size of total population.**Purposive Sampling**• Type of nonprobability sampling • Researcher uses personal judgment in subject selection. • Each subject chosen is considered representative of population. • Many qualitative studies use this technique.**Nonprobability Sampling Procedures**• Advantages • Time • Money • Disadvantages • Nonrandom • Not able to generalize findings**Learning Objective SixDetermine Which Sampling Technique To**Use In Various Types Of Research Studies**Research Studies**• Use voluntary subjects • Follow the ethics of research • Subjects must voluntarily agree. • Subjects may refuse to participate.**Research Data**• Based on voluntary responses from subjects • Biased sample occurs if subjects do not participate.**Volunteers As Subjects**• Sample selection varies. • Subjects volunteer for a study. • Researcher approaches subjects.**Random Sampling orRandom Assignment**• Random sampling • Each subject has equal probability of being included. • Random assignment • Procedure to ensure that each subject has equal chance

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