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Unformatted text preview: Roadmap : Experimental Design? Replication Random assignment Controls Applied Example: Marine reserves Statistics Definitions: Experiment: A study in which the investigator manipulates the factor of interest (independent variable), while keeping other factors constant, and observes the effect of the variable of interest (dependent variable). Only method for demonstrating causation Definitions: Statistical population: all possible units that we could study. Sample: the subset of the population from which we actually take data. Replicate: A sampling unit. Challenge : Four groups (e.g., treatments), from which well draw samples They may or may not differ in their true mean The first person to tell me which group(s) has the largest mean and which has the smallest can have $5. It costs $1 to play. If you can tell me the true mean for each Green Blue Yellow Red 10 20 30 What do you think now? Nonrandom Why was this hard? Why do we replicate? 22.5 21.5 13.0 22.5 True Means Key Elements : Replication: having >1 study unit. Random sampling: units that are sampled are representative of the population. Random assignment: units are assigned at random to treatments. Control: A treatment that provides a Why? Replication: Nature is variable Random sampling: Results apply to the population Random assignment: Treatment effects should be unbiased Independence & Pseudoreplicatio n True replicates must be independent (no shared error). Pseudoreplication: lack of independence among replicates Examples: heights of students at UF vs. FSU effects of temperature (incubators) Random assignme nt Take all units and randomly allocate them to treatments....
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 Spring '11
 Osenberg

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