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by: Ethel Hermiston


Ethel Hermiston
GPA 3.5


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This 11 page Class Notes was uploaded by Ethel Hermiston on Saturday September 12, 2015. The Class Notes belongs to STAT 4240 at University of Georgia taught by Wang in Fall. Since its upload, it has received 48 views. For similar materials see /class/202529/stat-4240-university-of-georgia in Statistics at University of Georgia.




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Date Created: 09/12/15
STAT 46240 SAMPLING AND SURVEY METHODS FALL 2011 Chapter 1 Introduction 1 Survey Sampling Goals and Basic Terminology Population The collection of all possible units of interest in any given study is called population It is usually the ideal population or universe to which research results are to be gener alized For example7 all adult population of the Us Cost typically prohibits us from interviewing all members of a population We therefore must select a sample from the population to interview Sample surveys are used to obtain information about a population by examining only a small fraction of the population Examples of elds in Which survey samples are used 0 Governments survey populations 7 The US government conducts health surveys and demographic surveys Bureau of Labor Statistics the unemployment rate is estimated by sampling 0 Public opinion polling Gallup Poll Predict election results 7 Harris Poll Predict election results CBSNew York times Poll Predict election results Nielsen Rating Estimates TV ratings 0 Business and Marketing 7 Which products to market 7 Where to market 7 How to advertise STAT 46240 SAMPLING AND SURVEY METHODS FALL 2011 0 There are rms which conduct surveys and information may be purchased from them on such questions Examples of such rms are Nielsen Retail Index Furnishes data on products such as foods cosmetics pharmaceutical beverages etc SAMl Selling Area Marketing INC Collects information on movement of products from warehouses and wholesalers 0 During audits of the nancial records of large companies sampling techniques may be used when examination of the entire set of records is impractical Three Elements of Statistical Study 0 Collecting Data Sample Surveys and Sampling 0 Describing and Presenting Data Graphical and Numerical descriptions 0 Drawing Conclusions from Data lnference estimation and test of hypothesis Basic Terminology 0 Observation unit OU An object on which a measurement is taken Err ample In human studies basic units are people 7 ln census basic units are households 7 ln length of life studies basic units are Light Bulbs 0 Target population TP The complete collection of observations we want to study Err ample Population of adults 18 in Clark County of Georgia UGA faculty staff students 7 Kids under 18 in Clark County of Georgia 7 Light bulbs produced by a company on one day 0 Sample A subset of the population STAT 46240 SAMPLING AND SURVEY METHODS FALL 2011 o Sampled population SP The collection of all possible observation units that might have been chosen in a sample the population from which the sample was taken 0 Sampling unit SU The unit we actually sample EMImple We may want to study individuals but not have a list of all individuals in the target population Instead households serve as the sampling units and the observation units are individuals in the household 0 Sampling frame SF The list of sampling units Examples 7 Lists of all residential street addresses 7 Phone numbers in speci c area codes 7 Maps 7 List of all hospitals in a county Remarks 1 Choice of SF determines SU eg Children7s health in city OU child SU depends on how survey is designed A SF list of house hold addresses from tax list SU household B SF school records SU child record C SF phone numbers land lines SU household phone number 2 Why not just have SF list of OU7s cost feasibility time convenience STAT 46240 SAMPLING AND SURVEY METHODS FALL 2011 2 Selection Bias Once you de ne TF target population then SP sampled population is determined by SF and NR nonresponse However rarely does TF SF TP SF Example TP children between 6 and 8 in Athens A SF list of households frorn Tax lists B SF school records for 1st 2nd and 3rd graders in public or church school TP TF SF 1 39139 Renters Nonpermanent homes Home schools Not all students Church schools in grade 13 are Tax list hhds no child 68 68 years A B When TP y SP you have selection bias That is if some part of the target population is not in the sampled population a bias called Selection Bias occurs For example in a survey to estimate per capita income CPI if transient people are ignored estimate will be biased upward A good sample will reproduce the characteristics of interest in the population as closely as possible STAT 46240 SAMPLING AND SURVEY METHODS FALL 2011 Probability sample Each unit in the population has a known probability of being se lected in the sample and a chance method such as using numbers from a random number table is used to choose the speci c units to be included in the sample Some Ways Selection Bias Can Occur O Misspeci cation of the Target Population 0 Failure to include the Target Population in the Sampling Frame7 also called under coverage 0 Selecting a convenient member of a population for a designated member not readily available7 also called convenience sample select convenient SUs o Nonresponse Failure to obtain responses from all those chosen in the sample dis torts the results of a survey7 typically non respondents differ from the respondents in some pronounced way 0 Allowing a sample to consist entirely of volunteers Radio7 TV7 or call in polls7 also called voluntapy sample SUs select themselves for sample C The investigator uses his or her judgment and knowledge of the population to select the units to be included in the population7 also called Judgment sample C Substitution Cannot research SU7 so choose a close one Example Sample 4000 students from students with special education7 10th grade in GA public schools SF students7 special education7 10th grade in GA public schools Methods 0 Check school records to see school complied with law 3980 out of 4000 records Response rate 39804000 995 o How do parentsguardians perceive the school is doing Dont know home addresses7 got list of name and addresses7 match with names in sample STAT 46240 SAMPLING AND SURVEY METHODS FALL 2011 Joe Smith Joseph Smith Joseph Lee Smith Out of 35007 only 900 responded Response rate 9003500 257 Exercise TP Students at local high school What is wrong in the following sampling method 0 Teachers are asked to select typical students for sample 0 Students absent7 take student in next seat 0 Sample selected from students in algebra classes 0 As in paper7 interested7 contact Joe Remark Note that large samples are generally considered good but if the sample is unrepresentative7 it can be quite bad The design of the survey is far more important than the absolute size of the sample STAT 46240 SAMPLING AND SURVEY METHODS FALL 2011 3 Measurement Error Measurement bias occurs when the measuring instrument has a tendency to record in one direction more often than the other It must be minimized in the design stage For example7 decide whether the plants on the boundary are to be counted Measurement biases are more common when dealing with people due to following reasons 0 People may not tell the truth 0 Lack of understanding of questions 0 Lack of proper account of events in memory 0 Variations in responses due to interviewers o Misreading questions7 or miss recording responses 0 Desire to impress the interviewer 0 Ordering and wording of questions have effects on responses STAT 46240 SAMPLING AND SURVEY METHODS FALL 2011 4 Questionnaire Design This is a very important step in conducting any survey Important Steps 1 Precise statement of goals of the survey Pose precise questions consistent with the goals7 design questionnaire to address these aspects 2 Test Questions Test questions before sending out the questionnaire Helps in understanding various possible interpretations of the questions 3 Keep the questions Simple and Clear Questions should be neither too lengthy nor too technical They should be easily understood by non experts 4 Questions should be speci c and not general 5 Open versus Closed questions Closed questions with well thought and researched categories elicit more accurate responses Open Question A question is open when no speci c response categories are pro vided Closed Question A question is closed when speci c response categories are pro vided 6 Report the actual question asked 7 Avoid leading or loaded questions These are questions that prompt or motivate the respondent to say what investigator wants to hear 8 Use choices rather than AgreeDisagree type questions 9 Ask only one concept in one question 10 Pay attention to question order effect Ask general questions rst then follow with speci c questions STAT 46240 SAMPLING AND SURVEY METHODS FALL 2011 5 Sources of Errors in Surveys The goal of a sample survey is to obtain as accurate as possible an estimate of a quantity about the population from the sample The source of errors in estimates from a survey can be divided into two major groups 0 Errors of nonobservatz39orz arise because the sampled elements make up only part of the target population 0 Errors of observation arise when recorded data deviates from the truth 51 Errors of nonobservation Sampling error Error that arises because we have taken one sample rather than looked at the whole population This is error that arises from the sample to sample variability of our estimates A key point is that in probability sampling7 the average magnitude of this error can be measured and the sampling error can be reduced by good survey designs and appropriate choice of sample size Coverage error Error that arises when the sampling frame does not match up per fectly with the target population For example7 the sample frame of all listed landline residential telephone numbers does not cover all households because some households have unlisted numbers and some households only have a cell phone Nonresponse Nonresponse arises in one of three ways 1 The inability to contact the sampled unit person or household7 for example An interviewer must not substitute a next door neighbor7 who just happens to be home at 3 pm7 for the person actually selected for the sample This type of substitution might lead to a survey that is biased because too many families with children or too many retired people or too many people who work nights are being interviewed E0 The inability of the person responding to come up with the answer to the question A survey on the economic impact of businesses on a community would be seriously biased if a few of the larger businesses do not know how much they spend on transportation STAT 46240 SAMPLING AND SURVEY METHODS FALL 2011 3 Refusal to answer People who are more interested or have stronger feelings about an issue are more likely to answer7 often biasing the results We will study how to try to reduce nonresponse and how to deal with remaining nonre sponse in the analysis in Chapter 8 Example of a survey with undercoverage and nonresponse The Literary Digest Poll Literary Digest was a popular magazine in the early 20th century that was well known for its poll that had correctly predicted the presidential election winner since its inception in 1916 to 1932 In 19367 the poll consisted of sending postcard questionnaires to 10 million people selected from subscribers to the Digest7 telephone directories and automobile owners The 2376523 returned post cards showed the Republican Alf Landon a winner over the Democrat Franklin Delano Roosevelt by 57 to 43 Final Returns in the Digest7s Poll of Ten Million Voters Landon7 1293669 Roosevelt7 972897 But7 Roosevelt won in a landslide7 63 to 37 The Literary Digest poll suffered from problems of coverage error and nonresponse 1 Coverage error The sampling frame overrepresented higher income people who were more likely to be Republican The readers of the Literary Digest tended to have above average incomes Furthermore7 in the 1930s7 owning a car or telephone was a luxury about 40 of households had residential phones E0 Nonresponse The anti Roosevelt voters felt more strongly than the pro Roosevelt voters and were more likely to respond Surveys that use voluntary responses to mailed questionnaires are often severely biased George Gallup made his fame by correctly predicting that Roosevelt would win based on a poll of 5000 people and also correctly predicting the results of the Literary Digest poll by choosing a random sample of a population chosen to match that of the Literary Digest poll STAT 46240 SAMPLING AND SURVEY METHODS FALL 2011 52 Errors of observation Once a person or object is in place and ready to be measured there are still more errors that can creep into the survey Interviewers have a direct and dramatic effect on the way a person responds to a question 0 Reading a question with inappropriate emphasis or intonation can force a response in one direction or another Most people who agree to an interview do not want to appear disagreeable and will tend to side with the view apparently favored by the interviewer especially on questions for which the respondent does not have a strong opinion 0 The race or gender of the interviewer may affect whether the respondent answers a question truthfully For example Schuman and Converse 1971 employed both white and African American interviewers to interview African American residents of Detroit To the question Do you personally feel that you can trust most white people some white people or none at all the response of 35 ofthose interviewed by a white person was that they could trust most white people The percentage was 7 for those interviewed by an African American person Respondents differ greatly in motivation to answer correctly and ability to do so 0 Each respondent must understand the entire question and be clear about the options for the answer This means questions must be clearly phrased and the questionnaire should not be too long Twenty two percent of Americans Say Holocaust May Not Have Occurred Akron Beach Journal 1993 based on Associated Press story The headline was based on the following Roper poll question Does it seem possible or does it seem impossible to you that the Nazi extermination of the Jews never happened 22 percent said it it seemed possible Another 12 said that they did not know Only 65 said it was impossible it never happened But the double negative in the question may have confused some respondents 0 Errors due to the respondent can be broadly classi ed into four categories 11


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