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Get out homework. Get out notes. Designing Samples. Section 5.1 Continued. Sample Designs. A simple random sample (SRS) of size n contains n individuals from the population chosen so that every set of n individuals has an equal chance of being selected. Sample Designs. - PowerPoint PPT Presentation

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Get out homework.

Get out notes.Section 5.1 ContinuedDesigning SamplesSample DesignsA simple random sample (SRS) of size n contains n individuals from the population chosen so that every set of n individuals has an equal chance of being selected.Every individual has an equal chance. Every set has an equal chance. 3Sample DesignsExample: SRS or not?I want a sample of nine students from the class, so I put each of your names in a hat and draw out nine of them.Does each individual have an equal chance of being chosen?

Does each group of nine people have an equal chance of being chosen?Yes for both. So yes on SRS.4Sample DesignsExample: SRS or not?I want a sample of nine students from the class but I know that there are three juniors and 17 seniors in class, so I pick one junior at random and eight seniors.Does each individual have an equal chance of being chosen?

Does each group of nine people have an equal chance of being chosen?

No, juniors more likely than seniors. No, because we cant have nine seniors. Not an SRS.5Sample DesignsBetter than a hat: Software can choose an SRS from a list of the individuals in a list.Not quite as easy as software, but still better than a hat: a table of random digits.Computers.Sample DesignsA table of random digits is a long string of the digits 0 9 with two properties: Each entry in the table is equally likely to be any of the ten digits 0 through 9The entries are independent of each other. (Knowing one part of the table tells you nothing about the rest of the table.)

TABLE B in the back of your book.Sample DesignsEach entry is equally likely to be 0 9Each pair of entries is equally likely to be 00 99Each triple of entries is equally likely to be 000 999And so on

Sample DesignsRead example 5.4 on p. 276Paper example9Sample DesignsA stratified random sample first divides a population into groups of similar individuals called strata. Then separate SRSs are chosen from each group (stratum) and combined to make the full sample.NOT an SRS because each group doesnt have an equal chance. Remember when I split the class into juniors and seniors before selecting? Same concept. The soundness of this design depends on how well you make first divisions. Do the example for stratified (class roster).10Cautions about samplesChoosing samples randomly eliminates human bias from the choice of sample, butWhat problems may remain? BrainstormGetting a list of the whole population. Selected cant be contacted. Throw some examples out11Cautions about samplesUndercoverageHaving an inaccurate list of the populationWho is excluded from a survey of households?Who is excluded from a telephone survey?Cautions about samplesNonresponseOccurs when selected individuals cannot be contacted or refuse to cooperate.Call and dont answer. Never home when contacted. Hang-ups.13ExamplesWhich problem (undercoverage or nonresponse) is represented?It is impossible to keep a perfectly complete list of addresses for the U.S. Census:Homeless people do not have addressesIn 1990, 35% of people who were mailed Census forms did not return them.Undercoverage for the first. Nonresponse for the second. What are other examples of undercoverage: students in dorms, inmates/prisoners14Response BiasResults may be influenced by behavior of either the interviewer or the respondent.Response BiasHow might response bias show up in these situations?A survey about drug use or other illegal behaviorQuestions asking people to recall events, like:Have you visited the dentist in the last six months?Response BiasThe wording of questions can often lead to bias:It is estimated that disposable diapers account for less than 2% of the trash in todays landfills. IN contrast, beverage containers, third-class mail, and yard wastes are estimated to account for 21% of the trash in landfills. Given this, in your opinion, would it be fair to ban disposable diapers?Response BiasDoes it seem possible or does it seem impossible to you that the Nazi extermination of the Jews never happened?22% said possibleDoes it seem possible to you that the Nazi extermination of the Jews never happened or do you feel certain that it happened?1% said possible18Inference about the PopulationEven if we can eliminate most of the bias in a sample, the results from the sample are rarely exactly the same as for the population.Each different sample pulls different individuals, so results will vary from sample to sampleResults are rarely correct for the populationInference about the PopulationSince we use random sampling, we can use the laws of probability (later chapters!)Well be able to figure out the margin of error (also in later chapters!)Inference about the PopulationJust know now: larger random samples give more accurate results than smaller samples.Homeworkp. 274 & 279 # 7 12

You do not need your book on Friday.

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