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1、Chapter 7,The Logic Of Sampling,Chapter Outline,Introduction A Brief History of Sampling Nonprobability Sampling The Theory and Logic of Probability Sampling,Chapter Outline,Populations and Sampling Frames Types of Sampling Designs Multistage Cluster Sampling Probability Sampling in Review,Political
2、 Polls and Survey Sampling,In the 2004 Presidential election, pollsters generally agreed that the election was “too close to call”. To gather this information, they interviewed fewer than 2,000 people.,Election Eve Polls - U.S. Presidential Candidates, 2004,Election Eve Polls - U.S. Presidential Can
3、didates, 2004,Election Eve Polls - U.S. Presidential Candidates, 2004,Bush Approval: Raw Poll Data,Question,One of the most visible uses of survey sampling lies in _. political polling probability sampling core sampling traditional polling,Answer: A,One of the most visible uses of survey sampling li
4、es in political polling.,Observation and Sampling,Polls and other forms of social research rest on observations. The task of researchers is to select the key aspects to observe (sample). Generalizing from a sample to a larger population is called probability sampling and involves random selection.,N
5、onprobability Sampling,Technique in which samples are selected in a way that is not suggested by probability theory. Examples include reliance on available subjects as well as purposive (judgmental), quota, and snowball sampling.,Types of Nonprobability Sampling,Reliance on available subjects: Only
6、justified if less risky sampling methods are not possible. Researchers must exercise caution in generalizing from their data when this method is used.,Types of Nonprobability Sampling,Purposive or judgmental sampling Selecting a sample based on knowledge of a population, its elements, and the purpos
7、e of the study. Used when field researchers are interested in studying cases that dont fit into regular patterns of attitudes and behaviors,Types of Nonprobability Sampling,Snowball sampling Appropriate when members of a population are difficult to locate. Researcher collects data on members of the
8、target population she can locate, then asks them to help locate other members of that population.,Types of Nonprobability Sampling,Quota sampling Begin with a matrix of the population. Data is collected from people with the characteristics of a given cell. Each group is assigned a weight appropriate
9、 to their portion of the population. Data should represent the total population.,Question,_sampling occurs when units are selected on the basis of prespecified characteristics. snowball quota purposive probability,Answer: B,Quota sampling occurs when units are selected on the basis of prespecified c
10、haracteristics.,Informant,Someone who is well versed in the social phenomenon that you wish to study and who is willing to tell you what he or she knows about it.,Probability Sampling,Used when researchers want precise, statistical descriptions of large populations. A sample of individuals from a po
11、pulation must contain the same variations that exist in the population.,Populations and Sampling Frames,Findings based on a sample represent the aggregation of elements that compose the sampling frame. Sampling frames do not always include all the elements their names imply. All elements must have e
12、qual representation in the frame.,A Population of 100 Folks,Sampling aims to reflect the characteristics and dynamics of large populations. Lets assume our total population only has 100 members.,Sample of Convenience: Easy but Not Representative,Types of Sampling Designs,Simple random sampling (SRS)
13、 Systematic sampling Stratified sampling,Representativeness,Representativeness - Quality of a sample having the same distribution of characteristics as the population from which it was selected. EPSEM - Equal probability of selection method. A sample design in which each member of a population has t
14、he same chance of being selected into the sample.,Question,_describes a sample whose aggregate characteristics closely approximate the aggregate characteristics of the population. exclusion probability sampling EPSEM representativeness none of these choices,Answer: D,Representativeness describes a s
15、ample whose aggregate characteristics closely approximate the aggregate characteristics of the population.,Population,The theoretically specified aggregation of study elements. Study population - Aggregation of elements from which the sample is actually selected. Element - Unit about which informati
16、on is collected and that provides the basis of analysis.,Random selection,Each element has an equal chance of selection independent of any other event in the selection process.,Sampling unit,Element or set of elements considered for selection in some stage of sampling.,Parameter,Summary description of a given variable in a population.,A Population of 10 People with $0$9,The Sampling Distribution of Samples of 1,In this example, the mean amount of money
