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Statistics & Econometrics Statistics & Econometrics Statistics & Econometrics Statistics & Econometrics Statistics & Econometrics Statistics & Econometrics Statistics & Econometrics Statistics & Econometrics Statistics & Econometrics Statistics for Economist Ch. 14 Sample Surveys 1.Population and Sample 2.Problems of Surveys 3.Sample Selection Methods 4.The Gallup Poll 5.Telephone Surveys 6.Probability Error Slide 2 Statistics & Econometrics Statistics & Econometrics Statistics & Econometrics Statistics & Econometrics Statistics & Econometrics Statistics & Econometrics Statistics & Econometrics Statistics & Econometrics Statistics & Econometrics STATISTICS 2/26 INDEX 1 Population and Sample 2 Problems of Surveys 3 Sample Selection Methods 4 The Gallup Poll 5 Telephone Surveys 6 Probability Error Slide 3 Statistics & Econometrics Statistics & Econometrics Statistics & Econometrics Statistics & Econometrics Statistics & Econometrics Statistics & Econometrics Statistics & Econometrics Statistics & Econometrics Statistics & Econometrics STATISTICS 3/26 1. Population and Sample population parameter Parameter estimation : estimator test statistic Population estimation : estimate test statistic sampling prior meaning - Random variable posterior meaning - Realized value inference stiatistic sample sampling Slide 4 Statistics & Econometrics Statistics & Econometrics Statistics & Econometrics Statistics & Econometrics Statistics & Econometrics Statistics & Econometrics Statistics & Econometrics Statistics & Econometrics Statistics & Econometrics STATISTICS 4/26 INDEX 1 Population and Sample 2 Problems of Surveys 3 Sample Selection Methods 4 The Gallup Poll 5 Telephone Surveys 6 Probability Error Slide 5 Statistics & Econometrics Statistics & Econometrics Statistics & Econometrics Statistics & Econometrics Statistics & Econometrics Statistics & Econometrics Statistics & Econometrics Statistics & Econometrics Statistics & Econometrics STATISTICS 5/26 2. Problems of Surveys Survey Roosevelt s percentage (%) The election result62 The Digest Prediction of the election result 43 Gallup s prediction of the Digest Prediction 44 Gallup s prediction for the election result 56 Prediction for the election of 1936 The Digest vs. Gallup Roosevelt wins Landon wins Roosevelt wins Gallup s overwhelming victory - Serious problems in sample selecting methods of the Digest. - Gallup even predicted the wrong prediction of the Digest Slide 6 Statistics & Econometrics Statistics & Econometrics Statistics & Econometrics Statistics & Econometrics Statistics & Econometrics Statistics & Econometrics Statistics & Econometrics Statistics & Econometrics Statistics & Econometrics STATISTICS 6/26 Problem (1)- sample selection bias Sample Selecting Methods of the Digest : -Survey using address from a tele- phone directory or club member lists -In those days, telephone or clubs were the symbols of the rich -the poor were excluded from the sample -The Digest got unfavorable results to Roosevelt who is popular to the poor -Systematic tendency(Sample selection bias) is related to sample selection. -The sample was not selected in a fair way. -The sample does not represent the population If there is sample selection bias, the sample doesn t represent the population. The problem is not solved just by increasing the magnitude of the sample. 2. Problems of Surveys Slide 7 Statistics & Econometrics Statistics & Econometrics Statistics & Econometrics Statistics & Econometrics Statistics & Econometrics Statistics & Econometrics Statistics & Econometrics Statistics & Econometrics Statistics & Econometrics STATISTICS 7/26 Problem (2)- non-response bias Sample Selection Procedure of The Digest -Only 2.4 million people bothered to reply, out of the 10 million who got the questionnaire -lower-income and upper-income people tend not to respond to questionnaires so the middle class was over-represented among respondents. Non-respondents can be very different from respondents. (non-respondent bias). -Respondents do not represent the sample Non-respondents may differ from the respondents. If the percentage of the non-respondents is high, you should consider the possibility of non-respondent bias. 2. Problems of Surveys Slide 8 Statistics & Econometrics Statistics & Econometrics Statistics & Econometrics Statistics & Econometrics Statistics & Econometrics Statistics & Econometrics Statistics & Econometrics Statistics & Econometrics Statistics & Econometrics STATISTICS 8/26 INDEX 1 Population and Sample 2 Problems of Surveys 3 Sample Selection Methods 4 The Gallup Poll 5 Telephone Surveys 6 Probability Error Slide 9 Statistics & Econometrics Statistics & Econometrics Statistics & Econometrics Statistics & Econometrics Statistics & Econometrics Statistics & Econometrics Statistics & Econometrics Statistics & Econometrics Statistics & Econometrics STATISTICS 9/26 3. Sample Selection Methods quota sampling and simple random sampling quota sampling simple random sampling The sample is hand-picked to resemble the population with respect to some key characteristics. Drawing at random without replacement -simple random sample But, there may be unintentional bias under part of the interviewers. But, practically it s difficult to do from the whole population. - Use multistage Cluster sampling Slide 10 Statistics & Econometrics Statistics & Econometrics Statistics & Econometrics Statistics & Econometrics Statistics & Econometrics Statistics & Econometrics Statistics & Econometrics Statistics & Econometrics Statistics & Econometrics STATISTICS 10/26 multistage cluster sampling MWNE W S 1 st stage : towns in North East 2 nd stage : wards 3 rd stage: an electoral district 4 th stage: households Sample Selection Method of Gallup Allocate the number of the sample according to the region. For each stage select samples by probability methods. Sample Selection Method of Gallup Allocate the number of the sample according to the region. For each stage select samples by probability methods. 3. Sample Selection Methods Slide 11 Statistics & Econometrics Statistics & Econometrics Statistics & Econometrics Statistics & Econometrics Statistics & Econometrics Statistics & Econometrics Statistics & Econometrics Statistics & Econometrics Statistics & Econometrics STATISTICS 11/26 sample selecting by a probability method - Simple random sampling, Multistage cluster sampling - features The interviewers have no discretion at all as to who they interview. There is a definite procedure for selecting the sample, and it involves the planned use of probability. -With the probability method it is possible to compute the probability that any particular individuals in the population will get into the sample. -For this makes the reproduced research by other people possible, we can construct the grounds for judging the significance of the results. 3. Sample Selection Methods Slide 12 Statistics & Econometrics Statistics & Econometrics Statistics & Econometrics Statistics & Econometrics Statistics & Econometrics Statistics & Econometrics Statistics & Econometrics Statistics & Econometrics Statistics & Econometrics STATISTICS 12/26 Multistage cluster sampling : deciding the sampling probability It is necessary to adjust the sampling probabilities according tothe magnitude of the sampling units. A Seoul B Pusan The probability of selecting a zone in Selecting 3 zones from Seoul & Pusan = 3/30 = 1/10 = the probability that A zone will be selected = the probability that B zone will be selected For the above formula, percentage of selecting Seoul = 2/3 percentage of selecting Pusan =1/3 The probability of selecting a zone in Selecting 3 zones from Seoul & Pusan = 3/30 = 1/10 = the probability that A zone will be selected = the probability that B zone will be selected For the above formula, percentage of selecting Seoul = 2/3 percentage of selecting Pusan =1/3 It is desirable to make the probabilities of sampling each individual same. 3. Sample Selection Methods Slide 13 Statistics & Econometrics Statistics & Econometrics Statistics & Econometrics Statistics & Econometrics Statistics & Econometrics Statistics & Econometrics Statistics & Econometrics Statistics & Econometrics Statistics & Econometrics STATISTICS 13/26 Multistage cluster sampling: probability adjustment of Gallup Towns Wards Precincts In each stage, decide the sampling probabilities according the rate of the households The probability of selecting one household is all the same regardless of the town, ward, or precincts to which the household belongs. If we just estimate the sampling probability by the number of households, not by the number of household members, some bias is generated. A member of more-member households is not likely to be selected rather than a member of less-members households. household bias 3. Sample Selection Methods Slide 14 Statistics & Econometrics Statistics & Econometrics Statistics & Econometrics Statistics & Econometrics Statistics & Econometrics Statistics & Econometrics Statistics & Econometrics Statistics & Econometrics Statistics & Econometrics STATISTICS 14/26 INDEX 1 Population and Sample 2 Problems of Surveys 3 Sample Selection Methods 4 The Gallup Poll 5 Telephone Surveys 6 Probability Error Slide 15 Statistics & Econometrics Statistics & Econometrics Statistics & Econometrics Statistics & Econometrics Statistics & Econometrics Statistics