Introduction to Experimental Design


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Introduction to Experimental Design 1 26 2009 Copyright 2009 Dan Nettleton 1 Terminology Experiment An investigation in which the investigator applies some treatments to experimental units and then observes the effect of the treatments on the experimental units by measuring one or more response variables 2 Terminology continued Treatment a condition or set of conditions applied to experimental units in an experiment Experimental Unit the physical entity to which a treatment is randomly assigned and independently applied 3 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 Observational Unit the unit on which a response variable is measured There is often a one toone correspondence between experimental units and observational units but that is not always true 4 Example 1 An experiment was conducted to study the effects of three soil moisture levels on gene expression in maize seedlings A total of 36 seedlings were grown in 12 pots with 3 seedlings in each pot The 3 soil moisture levels low medium and high were randomly assigned to the 12 pots with 4 pots for each soil moisture level After three weeks RNA was extracted from the aboveground tissues of each seedling Each of the 36 RNA samples was hybridized to a microarray slide to measure gene expression 5 A Cartoon Representation of the Experiment L L L L M M M M H H H H 6 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 7 Example 1 continued 1 Each of the three moisture levels represents a treatment 2 The moisture levels were randomly assigned to the pots so the pots are the experimental units A pot consisting of 3 seedlings is one experimental unit 3 Gene expression was measured for each seedling so the seedlings

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