Notes for Exam 2

Notes for Exam 2 - Ch. 8 Producing Data: Sampling Online...

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Ch. 8 – Producing Data: Sampling Online Notes Observational study: observes individuals and measures variables of interest but does not attempt to influence the responses. The purpose of an observational study is to describe some group or situation. Experiment: deliberately imposes some treatment on individuals in order to observe their responses. The purpose of an experiment is to study whether the treatment causes a change in the response. Two variables (explanatory variables or lurking variables) are confounded when their effects on a response variable cannot be distinguished from each other The population in a statistical study is the entire group of individuals about which we want information. A sample is a part of the population from which we actually collect information. We use a sample to draw conclusions about the entire population. A sampling design describes exactly how to choose a sample from the population. BAD SAMPLING The design of a statistical study is biased if it systematically favors certain outcomes. - bias: systematic error in the way a sample represents a population Voluntary response sample: consists of people who choose themselves by responding to a broad appeal. Voluntary response samples are biased because people with strong opinions are most likely to respond. Simple random sample (SRS) of size n consists of n individuals from the population chosen in such a way that every set of n individuals has an equal chance to be the sample actually selected. RANDOM DIGITS A table of random digits is a long string of the digits 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 with these two properties: 1. Each entry in the table is equally likely to be any of the 10 digits 0 through 9. 2. The entries are independent of each other. That is, knowledge of one part of the table gives no information about any other part. OTHER SAMPLING DESIGNS Probability sample is a sample chosen by chance. We must know what samples are possible and what chance, or probability, each possible sample has. Stratified random sample, first classify the population into groups of similar
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individuals, called strata. Then choose a separate SRS in each stratum and combine these SRSs to form the full sample. CAUTIONS ABOUT SAMPLE SURVEYS Undercoverage occurs when some groups in the population are left out of the process of choosing the sample. Nonresponse occurs when an individual chosen for the sample can’t be contacted or refuses to participate. Response bias example: People know that they should take the trouble to vote, for example, so many who didn’t vote in the last election will tell an interviewer that they did. Wording of questions is the most important influence on the answers given to a sample survey. Confusing or leading questions can introduce strong bias, and even minor changes in wording can change a survey’s outcome Large random samples give more accurate results than smaller samples. Class notes – 10/15/07
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Notes for Exam 2 - Ch. 8 Producing Data: Sampling Online...

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