Please get out materials for notes. Make sure to grab a outline at the front desk. 22 23 Introductions/ Expectations Sampling Design SRSs (4.1) 29 Observational Study vs. Experiment (4.2) 5 No School Labor Day 24/25 Sampling Design (4.1) Bias, Random Digits,
Bear Hunt Visual Bias Lab 26 Pep Rally Get Textbooks if needed Federalist Paper activity Assign take-home Quiz 4.1 30 Take-home quiz due Experimental Design (4.2) 31/1 Experimental Design (4.2) Experimental Design WKST 2 Quiz 4.2 Assign Current Event Project
6 Challenges of establishing Causation (4.3) 7/8 FRAPPY! Review Chapter 4 9 Test 1: Chapter 4 Designing Studies The Current Event Project will be due on Tuesday, September 13th. Ch. 4: Designing Studies S EC T I O N 4. 1 S A M P L ES & S URV EY S Samples and Surveys Learning Objectives After this section, you should be able to: IDENTIFY the population and sample in a statistical study. IDENTIFY voluntary response samples and convenience
samples. EXPLAIN how these sampling methods can lead to bias. DESCRIBE how to obtain a random sample using slips of paper, technology, or a table of random digits. DISTINGUISH a simple random sample from a stratified random sample or cluster sample. Give the advantages and disadvantages of each sampling method. EXPLAIN how undercoverage, nonresponse, question wording, and other aspects of a sample survey can lead to The Practice of Statistics, 5th Edition 4 Population, Census, and Sample Population Population -- aa statistical statistical study study is is the the entire entire group group of
of individuals individuals we we want want information information about. about. Census Census -- collects collects data data from from every every individual individual in in the the population. population. Sample Sample -- aa subset subset of of individuals individuals in in the the population population from from which
which we we actually actually collect collect data. data. Population Sample Collect data from a representative Sample... Make an Inference about the Population. Example on P. 210 Identify the population and the sample in each of the following settings: a) A furniture maker buys hardwood in large batches. The supplier is supposed to dry the wood before shipping (wood that isnt dry wont hold its size and shape). The furniture maker chooses five pieces of wood from each batch and tests their moisture content. If any piece exceeds 12% moisture content, the entire batch is sent back.
b) Each week, the Gallup Poll questions a sample of about 1500 adult U.S. residents to determine national opinion on a wide variety of issues. The Idea of a Sample Survey Choosing a Sample Step 1: Define the population we want to describe. Step 2: Say exactly what we want to measure. Sample survey - a study that uses an organized plan to choose a sample that represents some specific population. Step 3: Decide how to choose a sample from the population. How to Sample Badly How can we choose a sample that we can trust to represent the population? Choosing Choosing individuals individuals from from the the population population who who are
are easy easy to to reach reach results results in in aa convenience convenience sample. sample. Convenience samples often produce unrepresentative datawhy? The The design design of of aa statistical statistical study study shows shows bias bias ifif itit would would consistently consistently underestimate underestimate or or consistently consistently overestimate
overestimate the the value value you you want want to to know. know. How to Sample Badly A A voluntary voluntary response response sample sample consists consists of of people people who who choose choose themselves themselves by by responding responding to to aa general
general invitation. invitation. Voluntary Voluntary response response samples samples show show bias bias because because people people with with strong strong opinions opinions (often (often in in the the same same direction) direction) are are most most likely likely to to respond. respond.
AP Exam Tip, P. 212 If youre asked to describe how the design of a study leads to bias, youre expected to do the following: Identify a problem with the design. 2. Explain how this problem would lead to an underestimate or overestimate. 1. Check Your Understanding P. 213 For each of the following situations, identify the sampling method used. Then explain how the sampling method could lead to bias. 1. A farmer brings a juice company several crates of oranges each week. A company inspector looks at 10 oranges from the top of each crate before deciding whether to buy all the oranges. Convenience sampling This could lead to the inspector overestimating the quality of the
oranges if the farmer puts the best oranges on top. Check Your Understanding P. 213 For each of the following situations, identify the sampling method used. Then explain how the sampling method could lead to bias. 2. The ABC program Nightline once asked whether the United Nations should continue to have its headquarters in the U.S.. Viewers were invited to call one telephone number to respond Yes and another for No. There was a charge for calling either number. More than 186,000 callers responded, and 67% said No. Voluntary response sampling those who are happy that the UN has its headquarters in the U.S. already have what they want and so are less likely to respond. The proportion who answered No in the sample is likely to be higher than the true proportion in the U.S. who would answer No. How to Sample Well: Simple Random Sampling
A sample chosen by chance rules out both favoritism by the sampler and self-selection by respondents. Random sampling involves using a chance process to determine which members of a population are included in the sample. A A simple simple random random sample sample (SRS) (SRS) of of size size n n is is chosen chosen in in such such aa way way that that every every group group of of nn individuals individuals in in the the population
population has has an an equal equal chance chance to to be be selected selected as as the the sample. sample. In practice, people use random numbers generated by a computer or calculator to choose samples. If you dont have technology handy, you can use a table of random digits (Table D in book). Homework Due Tuesday P. 229 # 1 9 odd
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