Chapter 8 Statistical Principles of Design Fall 2010 Experimental Design Many interesting questions in biology involve relationships between response variables and one or more explanatory variables Biology is complex and typically many potential variables both those measured and included in an analysis and those not measured may influence the response variable of interest A statistical analysis may reveal an association between an explanatory variable and the response variable It is very difficult to attribute causal effects to observational variables because the true causal influence may affect both the response and explanatory variable However properly designed experiments can reveal causes of statistical associations The key idea is to reduce the potential effects of other variables by designing methods to gather data that reduce bias and sampling variation Case Studies We will introduce aspects of experimental design on the basis of these case studies An education example An Arabidopsis fruit length example A starling song length example A dairy cow nutrition study A weight loss study Education Example Example A researcher interested in biology education considers two different curricula for high school biology Students in one school follow a standard curriculum with lectures and assignments all from a textbook Students in a second school have the same lectures and assignments but spend one day each week participating in small groups in an inquiry based research activity Students from both schools are given the same exam Students with the standard curriculum score an average of 81 2 and the group of students with the extra research score an average of 88 6 hypothesis tests both a permutation test and a two independent sample t test have very small p values indicating higher mean scores for the extra research group Is there evidence to indicate that the supplemental inquiry based research activity increases exam scores Arabidopsis Example Example A researcher conducts an experiment on the plant Arabidopsis thaliana that examines fruit length a gene from a related plant is introduced into the genomes of four separate Arabidopsis plants each of these plants is the progenitor of a transgenetic line an additional Arabidopsis plant is included in the experiment but does not have the trans gene introduced these five plants represent the T1 generation each T1 plant is grown self fertilized and seed is collected a sample of 25 seeds from each plant are potted individually grown and self fertilized these plants are the T2 generation the length of a sample of ten fruit is measured for each T2 plant Starling Song Example Starlings are songbirds common in Wisconsin and elsewhere in the United States Male starlings sing in the spring from a nest area when they attempt both to attract females as potential mates and to keep other males away Male starlings sing in the fall when they are in flocks of other male birds It is difficult to categorize a single song as spring like or fall like but characteristics of song can be different at the two times One simple song characteristic is the length of the song In an experiment a researcher randomly assigned 24 starlings into two groups of 12 Starling Song cont Example All measurements are taken in animal observation rooms in a research laboratory The spring group was kept in a spring like environment with more light a nest box and a nearby female starling The male group was kept in a fall like environment with less light no nest boxes and in the proximity of other male birds Each bird was observed and recorded for ten hours birds sang different numbers of songs and the length of each song was determined Each bird sang from between 5 and 60 songs In the actual study characteristics of the songs beyond their length were of greater importance Dairy Cattle Diet Example Example In a study of dairy cow nutrition researchers have access to 20 dairy cows in a research herd Researchers are interested in comparing a standard diet with three other diets each with varying amounts of alfalfa and corn In the experiment the cows are randomly assigned to four groups of 5 cows each Each group of cows receives each of the four diet treatments for a period of three weeks no measurements are taken the first week so the cow can adjust to the new diet The diets are rotated according to a Latin Square design so that each group has a different diet at the same time Response variables include milk yield and abundance of nitrogen in the manure Latin Square Design Diets are named A B C and D Each group of cows gets all four diets but in different orders Group 1 2 3 4 First A C B D Time Period Second Third Fourth B C D A D B D A C C B A Weight loss Study Example Researchers in the Department of Nutrition recruited 60 overweight volunteers to participate in a weight loss study Volunteers were randomly divided into two treatment groups All subjects received educational information about diet One treatment group was instructed to count and record servings of each of several food types each day the other treatment group was instructed to count and record calories consumed each day Subjects were not aware of the instructions given to members of the other group Confounding Definition A confounding variable is a variable that masks or distorts the relationship between measured variables in a study or experiment Two variables are said to be confounded if their effects on a response variable cannot be distinguished or separated 1 What are possible confounding variables that may explain the differences in test scores in the education example 2 What potential confounding factors are researchers trying to avoid with the Latin square design for the dairy cow nutrition study 3 What are potential confounding factors in the weight loss example Experimental Artifacts Example An early experiment finds that the heart rate of aquatic birds is higher when they are above water than when they are submerged Researchers attribute this as a physiological response to conserve oxygen In the experiment birds are forcefully submerged to have their heart rate measured A later experiment uses technology that measures heart rate when birds voluntarily submerge and finds no difference in heart rates between submerged and above water groups This suggests that the stress induced by forceful submersion rather than submersion itself caused the lowering of heart rate in the birds Definition A experimental
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