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5.1B

5.1B. Simple Random Samples. Choosing a sample by chance. Voluntary response sample- people choose whether to respond Convenience sample- the interviewer makes a choice In both cases personal choice produces bias.

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5.1B

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  1. 5.1B Simple Random Samples

  2. Choosing a sample by chance • Voluntary response sample- people choose whether to respond • Convenience sample- the interviewer makes a choice • In both cases personal choice produces bias. • A sample chosen by chance allows neither favoritism by the sampler nor self-selection by respondents.

  3. Equal chance to be chosen • Choosing a sample by chance attacks bias by giving all individuals an equal chance to be chosen • Placing names in a hat (the population) and draw out a handful (the sample) is the simples example of simple random sampling.

  4. TABLE B • Table of random digits • Contains digits 0-9 • Entries are independent of each other • Knowledge of one part of the table gives no information about any other part • Each entry is equally likely to be any part of the 10 possibilities 0, 1,…..,9. • Each pair… any of the 100 possible pairs 00, 01,…,99. • Each triple…any of the 1000 possibilities 000, 001,…,999 and so on.

  5. Choosing an SRS • Step one: LABEL. Assign a numerical label to every individual in the population. • Labels can be assigned in any convenient manner • All labels must have the same number of digits. • One digit for population of up to 10 members, 2 digits for population 11 to 100, three for 101 to 1000 and so on. • As standard practice we read the table across rows. • Step two: TABLE. Use Table B to select labels at random.

  6. Probability sample • A sample chosen by chance. • Some probability sampling designs (such as SRS) give each member an equal chance to be selected. • Not always true in more elaborate sampling designs. • In every case, the use of chance to select the sample is the essential principal of statistical sampling.

  7. Stratified random sample • To select a stratified random sample, first divide the population into groups of similar individuals, called strata. • Then choose a separate SRS in each stratum and combine these SRSs to form the full sample.

  8. Assignment Pg. 279 #5.9-5.12

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