统计第二章.pdf

Page 3 types of experiments completely randomised

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Page 3 Types of Experiments Completely Randomised Design The treatments are allocated entirely by chance to the experimental units. Example: Suppose 8 men were involved in the aspirin study. Randomly allocate treatments to the men. 1 2 3 4 5 6 7 8 A A A P P A P P Random Numbers 11369 23569 26339 42774 79623 92280 93246 Randomised Block Design Group ( block ) experimental units by some known factor and then randomly allocate treatments within each block. Example: Suppose 12 men were involved in the aspirin study and it is known that age (especially being 65 or older) is a risk factor for heart attacks. Divide the men into two blocks and then randomly allocate treatments to men within each block. Age: 44 47 50 53 55 55 62 63 65 69 73 74 1 2 3 4 5 6 7 8 1 2 3 4 A P A P A A P P A P P A Random Numbers 65169 37186 05143 32004 74092 760116 50572 What are some other risk factors for heart attack?
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Chapter 2 Page 4
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Chapter 2 - Notes Observational Studies What is an observational study? A study where the data comes from observing and recording things as they are in the world, or as they unfold over time, without the investigator actively changing anything is called an observational study. A study in which researchers simply compare units at each level of the factor of interest is an example of an observational study. We use observational studies when we are interested in studying the effect of a factor of interest on individuals or units We compare the units that happen to have received each of the levels of the factor of interest. We measure the response for each unit under an observed factor level. The aim is to make comparisons fair — try to make groups as similar as possible except for the factor of interest. Observational studies should use some form of random sampling to obtain representative samples. Observational studies are useful for identifying possible causes of effects, but they cannot reliably establish causation. Types of observational studies include: 1. Cross-sectional A study which observes a group of individuals or units at a point in time. It is a descriptive study, providing a “snapshot” at a particular point in time. 2. Longitudinal A study which observes the same group of individuals or units over a long period of time. A longitudinal study is comprised of a series of cross-sectional studies. E.g., The Dunedin Longitudinal Study is a study based in Dunedin, New Zealand. 1037 children born between 1 April 1972 and 31 March 1973 initially took part in the study. The study is still continuing. Refer: Chance Encounters, pages 24 – 28 Page 5 Experiments What is an experiment? A study in which the researcher controls (or manipulates or changes) the conditions experimental units experience is called an experiment. A study in which the researchers determine which units (people) receive which treatments is an example of an experiment.
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  • Spring '13
  • 101
  • researcher, observational studies

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