VCU STAT 210 - Lecture3 (32 pages)

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Lecture3



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Lecture3

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Pages:
32
School:
Virginia Commonwealth University
Course:
Stat 210 - Basic Practice of Statistics
Basic Practice of Statistics Documents
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STAT 210 Lecture 3 Producing Data August 30 2017 Practice Problems Pages 38 through 42 Relevant problems II 1 and II 11 Recommended problems II 1 and II 11 Additional Reading and Examples Pages 26 through 29 Particularly read 7 on page 29 Top Hat Motivating Example It is of interest to determine what percentage of all VCU students who when asked to list three things that they enjoy doing include working out playing sport gym on their list Articles There are two articles one on pages 11 through 14 and one on pages 15 and 16 Read the article on pages 15 and 16 of the Supplement and answer the three clicker questions related to this Top Hat 3 Articles Outside of class read the article on pages 11 14 and think about the population of interest the sample and any parameters and statistics Producing Data To perform any type of statistical analysis or statistical inference we need data How do we get this data Sampling Procedures We select a sample of the population and only measure or contact the subjects in the sample Experimental Designs A treatment is imposed on the experimental units or subjects and a response is observed You then can compare the effect that the treatment is having on the response A Sampling Designs A population is a large set of subjects that the researcher wants to make statements about Example all VCU students The problem is that in many cases time cost or inconvenience make it impossible to contact or measure every member of the population A Sampling Designs Instead we obtain a representative sample of the population and use the data in the sample to make inferences about the population Example We can easily contact a sample of 200 VCU students Representative Sample The sample should be as representative of the population as possible meaning that the characteristics of the sample should mimic the characteristics of the population Example If the population is 60 female then the sample should be roughly 60 female as well if we think a person s sex will influence the response Bias Bias exists when some subjects or outcomes are systematically favored over others A sample which is representative of the population should be free of bias If the sample is not representative then the results will be biased in favor of the responses of those which are overrepresented Selection Bias Selection bias occurs when one or more types of subjects are systematically excluded from the sample Selection Bias Selection bias occurs when one or more types of subjects are systematically excluded from the sample Example Suppose the population consists of all students at this university and of interest is the opinion of students regarding an increase in student fees to pay for an expansion of the basketball arena Contacting all students for their opinions is not possible so instead a sample of 200 students in attendance at a basketball game is selected and they are asked their opinion This would likely create selection bias because the opinions of students not attending the basketball game were not included Most likely those attending the basketball game would be more willing to support an increase in fees for this purpose while those who do not have an interest in basketball would be less likely to support such a fee increase Selection Bias When selection bias exists the results from the sample can only be inferred to part of the population The inference cannot be made to the entire population but only a subset of the population This is referred to as undercoverage Example Instead of using the 200 responses to make inferences about the opinion of all students the 200 responses can only be used to make inferences about the opinion of all students who attend basketball games Opinions of students who do not attend the basketball games are not represented and hence inference cannot be made about these students this is the undercoverage Nonresponse Bias When an individual randomly chosen to be a part of the sample cannot be contacted or fails or refuses to respond then we have a nonresponse bias This is often a big problem in surveys or polls in which the person either throws the survey away or refuses to answer the questions of those conducting the poll Nonresponse Bias Example Suppose the U S Government is interested in the opinions of citizens regarding the rights of illegal aliens in this country A sample of 500 citizens is chosen such that a representative number of citizens from each racial and ethnic group is included However if all the Latino Americans refuse to respond then we would have nonresponse bias if their opinions are different from those of other racial or ethnic groups Response Bias When respondents give inaccurate information or if the interviewer influences the subject to respond in a certain way due to the way the questions are phrased this is response bias This is especially a concern with legal or social behavior issues Response Bias When respondents give inaccurate information or if the interviewer influences the subject to respond in a certain way due to the way the questions are phrased this is response bias This is especially a concern with legal or social behavior issues Example 1 Have you ever committed a crime that involved inflicting injury to the body of another person Due to social and legal concerns the number of no responses will likely be higher than the actual number of people for which the correct response is no Response Bias When respondents give inaccurate information or if the interviewer influences the subject to respond in a certain way due to the way the questions are phrased this is response bias This is especially a concern with legal or social behavior issues Example 2 Goal is to determine opinions about how safe people feel flying different airlines Consider the question Do you feel safer flying on Delta Airlines as opposed to one of the other airlines This question would cause a response bias in favor of Delta Airlines over other airlines Top Hat Haphazard Samples A haphazard sample involves selecting a sample by some convenient mechanism that does not involve randomization A mall survey in which questionnaires are distributed to people as they walk through the mall or a campus survey in which students are questioned as they walk across campus are two examples of haphazard samples Volunteer Response Samples A volunteer response sample exists when subjects volunteer to be part of the study Examples include telephone call in polls internet


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