09%20Experimental%20Design%202_07_08

09%20Experimental%20Design%202_07_08 - Terminology...

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1 1 Introduction to Experimental Design 2/07/2008 Peng Liu 2 Terminology ± Experiment – An investigation in which the investigator applies (assigns) some treatments to experimental units and then observes the effect of the treatments on the experimental units by measuring one or more response variables. ± The assignment rule, or, the experimental design , specifies which experimental units are to be observed under which treatments. 3 Terminology (continued) ± Treatment – a condition or set of conditions applied to experimental units in an experiment. ± Experimental Unit - the physical entity to which It is the smallest division of experimental material (e.g. land, plant, animal, etc) that may receive different treatments. 4 Terminology (continued) ± Response Variable – a characteristic of an experimental unit that is measured after treatment and analyzed to assess the effects of treatments on experimental units. (e.g. yield, gene expression level, etc.) ± Observational Unit - the unit on which a response variable is measured. There is often a one-to-one correspondence between experimental units and observational units, but that is not always true.
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2 5 Experiment vs Observational Study ± In some cases, the treatments can not be assigned to experimental units by some rule. ± For example, to study the effects of smoking on cancer with human objects as the experimental units Æ neither ethical nor possible ± We can still gather data and analyze them, but can not draw conclusion of causation . ± This type of study is called observational study and is not an experiment. 6 Example 1 ± An experiment was conducted to study the effects of three types of feed (A, B, and C) on gene expression in pigs. ± A total of 36 pigs were housed in 12 pens with 3 pigs in each pen. ± The 3 feed types were randomly assigned to the 12 pens with 4 pens for each feed type. ± After three weeks, 36 Affymetrix GeneChips were used to measure gene expression in muscle tissues from the 36 pigs. One GeneChip was used for each pig. 7 Example 1 (continued) 1. Name the treatments in this experiment. 2. Name the experimental units in this experiment. 3. Name the observational units in this experiment. 4. Name the response variable or variables in this experiment. 8 Example 1 (answers)
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3 9 Terminology (continued) ± Response Variable – a characteristic of an experimental unit that is measured after treatment and analyzed to assess the effects of treatments on experimental units. ± Explanatory Variable – a variable that can potentially be used to explain variation in a response variable. 10 Terminology (continued) ± Factor – an explanatory variable that can take any one of two or more values. ± Levels
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09%20Experimental%20Design%202_07_08 - Terminology...

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