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# Test%203%20Notes - Chapter Four Gathering Data 4.1 Should...

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Page of 53 Chapter Four – Gathering Data 4.1 Should we experiment or observe? There are two basic ways to gather data: 1. Observational Study 2. Experiment Difference between experiments and observational studies: Experiments attempt to manipulate or influence the subjects in an experiment. Properly designed experiments can be used to prove causation (that one variable CAUSES the other to change). In using experiments, subjects can be randomly assigned to groups. Observational studies simply measure characteristics of the subjects without attempting to manipulate or influence the subjects. Observational studies cannot be used to prove causation, they can only say that the variables are related to one another. In using an observational study, subjects cannot be randomly assigned to groups. The advantages of using an experiment are that you can prove causation and apply randomization, while an observational study can only prove association of two variables and not causation. Page of 53 Example: Decide whether the following are experiments or observational studies: 1. Rats with cancer are divided into 2 groups. One group receives 5 mg a day of an experimental drug that is thought to fight cancer, the other group receives 10 mg a day of the same drug. After 2 years, the spread of cancer is measured in both groups. Subjects? Experiment or observational study? 2. A poll is conducted in which 500 people are asked whom they plan to vote for in the upcoming election. Subjects? Experiment or observational study? Page of 53 4.2 – 4.3 Observational Study vs. Experiment TYPES OF OBSERVATIONAL STUDIES: Cross-Sectional Study – a study that attempts to take a cross section of the population at the current time. Ex. A survey that asks “Who is your current favorite singer?” Retrospective Study – a study that is “backward looking”. Ex. A study to see if there is an association between cell phone usage and brain cancer. We can form a sample of subjects with brain cancer and a sample of subjects without brain cancer and compare the past use of cell phones for both groups. Prospective Study – a study that is “forward looking.” Ex. A study that asks “How many hours a week do you watch television?” knowing that they will ask the same people this question in one year. Case-control study – a retrospective study in which subjects who have a response outcome of interest (the cases) and subjects who have the other response outcome (the controls) are compared on an explanatory variable. Ex. The brain cancer-cell phone usage example. Page of 53 Sources of Potential Bias in Sample Surveys 1. Sampling Bias – occurs from using nonrandom samples or not using a large enough sample frame (undercoverage). Example: In an election poll, if we just took the first 30 people that walked up to us, rather than randomly selecting people to poll, this might result in sampling bias. (This would be called convenience sampling, which we will talk more about later.) Or, if we just

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## This note was uploaded on 09/10/2011 for the course STAT 2000 taught by Professor Smith during the Spring '08 term at UGA.

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Test%203%20Notes - Chapter Four Gathering Data 4.1 Should...

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