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QM 2113 - Spring 2002 Business Statistics Sampling Concepts
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Student Objectives Discuss the issues involved in data gathering Summarize various methods of sampling Define what a “random” sample is Explain the concept of sampling error Calculate the expected sampling error for any given random sampling process Determine probabilities associated with the sampling distribution of the mean Explain the Central Limit Theorem
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But Before We Get Started... Turn in your work: – Midterm exam – Collect homework Chapters 1 & 6 Looking only for “reasonable attempt” Midterm exam comments – Clearly not enough prior practice! First or second time for some techniques Should –Work homework when assigned –Rework until proficient – Preparation: every conceivable analysis Salary, age, salary vs. age,... Gender, computers, gender vs. computers,... And so on...
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Basics of Data Gathering Remember the purpose: decision making concerning some population or process Therefore, the sample must be capable of supporting inferences upon which those decisions are to be based Sampling approaches – Nonstatistical: based upon judgement – Convenience: can result in bias – Statistical: provide foundation for inference Randomness: each element in the process or population has an equal chance of being selected Note: random ≠ representative
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Recall: Parameters versus Statistics Descriptive numerical measures calculated from the entire population are called parameters. – Quantitative data: and – Qualitative data: (proportion) Corresponding measures for a sample are called statistics. – Quantitative data: x-bar and s – Qualitative data: p
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Random Sampling We need to get a little theoretical for a while here, but not too long Consider a set of numbers; call it our population of interest – The average summarizes where on a number line the values tend to be – The standard deviation provides a measure of how different the values are Suppose we select a subset (i.e., a sample) – It can also be described by the average and standard deviation – But those values aren’t the same
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Sampling Process Population or Process Sample Parameter Statistic
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Some Things to Note Sampling error – Difference between parameter and statistic – Exact amount always unknown, but... If we had taken a different sample, the statistic would have been a different value Thus, the outcomes (i.e., statistics) of sampling are random variables themselves – Expected value for x-bar – Expected variation for x-bar
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It Turns Out That... The expected value for x-bar is the population or process average (i.e., ) The expected variation in x-bar from one sample average to another is – The average (expected) sampling error – Known as the standard error of the mean – Summarized by the sampling distribution Possible values for x-bar Probabilities associated with those values If the sample size is large enough, the sampling distribution is normal (CLT)
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Let’s Work Some Problems Exercises assigned – Exercise 1 – Exercise 2 – Exercise 8 – Exercise 12 Consider the implications Why is this important? INFERENCE!!!
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Homework Rework (as necessary) exercises assigned from Chapter 6 Finish reading Chapter 6 Additional problems from Chapter 6 Rethink the midterm exam and bring one question to class
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