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1 CHAPTER 3 – EXPERIMENTAL AND SAMPLING DESIGN Overview of how to answer a research question: 1. Pick a specific question you want to answer. 2. Decide on your population. 3. Select a sampling design and gather your sample. The choices for sampling designs for this class are: voluntary response (the only one not random, not the best) simple random sample stratified random sample multistage sample catch-and-release sample 4. Decide whether to conduct an observational study or an experiment. If observational study, just state the sampling design. If experiment, the choices for experimental designs for this class are: completely randomized design block design matched pairs 5. Choose your response variable and your explanatory variables. Decide on your treatments (for experiments). 6. Collect the data. 7. Analyze your data using either exploratory data analysis (looking for trends/relationships in the actual data) or formal statistical inference (answering statistical questions with a known degree of confidence). 8. State your conclusions. What can go wrong? Bias (response bias, nonresponse, undercoverage) Variability Poor experimental design (not using a control, not randomizing, not replicating) Other (poor choice of sampling design, date of survey) Think about why your variables are related. Causation is not the same thing as association! Is the relationship between your variables based on: Causation Confounding Common response Principles of Ethical Experiments square4 Planned studies should be reviewed by a board to protect subjects from harm. square4 All subjects must give their informed consent before data are collected. square4 All individual data must be kept confidential. Only summaries can be made public. (Anonymity is not the same as confidentiality.)
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2 VOCABULARY Population : The entire group of units or individuals about which we desire information. Sample : The part of the population selected to be measured or observed in order to gather data for analysis. Census : An attempt to contact every individual in the entire population. Response Variable : Variable we are interested in studying. Unit : An individual person, animal or object upon which the response variable is measured. Units are called “individuals” when they refer to people. EXAMPLE A forester is interested in determining the total number of trees that are planted on tree farms in Montana. The forester believes the number of trees varies with the size of the tree farm. He divides all such farms into four classes depending on their size. From each class, he selects a sample of 15 farms. He counts and records the total number of trees for each of the selected tree farms. Unit: Population: Sample: Response variable: MAJOR IDEA We are interested in one or more variables associated with a population of units. Because it is impossible or too expensive to measure the variables of interest on all the units in the population, we only measure the variables on a subset or a sample of units. We use the sample to draw conclusions about the population.
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