Unit 5-1

# Unit 5-1 - Sampling Design Design How do we gather data?...

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Sampling Sampling Design Design

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How do we gather data? How do we gather data? Surveys  Opinion polls Interviews Studies Observational Retrospective (past) Prospective (future) Experiments
Population Population the entire group of individuals that we want information about

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Census Census a complete count of the population
How good is a How good is a census? census? Do frog fairy tale . . . The answer is 83!

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Why would we not use Why would we not use a census all the time? a census all the time? 1) Not accurate 2) Very expensive 3) Perhaps impossible 4) If using destructive sampling, you would destroy population Breaking strength of soda bottles Lifetime of flashlight batteries Safety ratings for cars Look at the U.S. census – it has a huge amount of error in it; plus it takes a long to compile the data making the data obsolete by the time we get it! Suppose you wanted to know the average weight of the white-tail deer population in Texas – would it be feasible to do a census? Since taking a census of any population takes time, censuses are VERY costly to do!
Sample Sample A part of the population that we actually examine in order to gather information Use sample to generalize to population

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Sampling Sampling design design refers to the method used to choose the sample from the population
Sampling frame Sampling frame a list of every individual in the population

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consist of n individuals from the population chosen in such a way that every individual has an equal chance of being selected every set of n individuals has an equal chance of being selected Simple Random Simple Random Sample (SRS) Sample (SRS) Suppose we were to take an SRS of 100 SWH students – put each students’ name in a hat. Then randomly select 100 names from the hat. Each student has the same chance to be selected! Not only does each student has the same chance to be selected – but every possible group of 100 students has the same chance to be selected! Therefore, it has to be possible for all 100 students to be seniors in order for it to be an SRS!
Stratified random Stratified random sample sample population is divided into homogeneous groups called strata SRS’s are pulled from each strata Homogeneous groups are groups that are alike based upon some characteristic of the group members. Suppose we were to take a stratified random sample of 100 SWH students. Since students are already divided by grade level, grade level can be our strata . Then randomly select 50 seniors and randomly select   50 juniors.

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Systematic Systematic random sample random sample select sample by following a systematic approach randomly select where to begin Suppose we want to do a systematic random sample of SWH students - number a list of students
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## This note was uploaded on 12/09/2011 for the course STATS 221 taught by Professor Nielson during the Fall '10 term at BYU.

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Unit 5-1 - Sampling Design Design How do we gather data?...

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