Doing Experiments on PopulationsReading AssignmentTypes of SciencePractice versus scienceGeneralityConstants, Variables, ParametersTypes of VariablesPopulationsStatisticsCausation & CorrelationExperiments look for outcomes following actionsWhat motivates one to do an experiment?Does doing something particular work (better than chance)?The HYPOTHESISMultiple ExplanationsEXPERIMENTAL DesignExperimental DesignDrug EffectivenessMedieval Science LabContingency Chi-squaredProblemVocabularyx1 09 Experimental Design 1Doing Experiments on Doing Experiments on PopulationsPopulationsWhat are the ways science generates What are the ways science generates knowledge (=truth)?knowledge (=truth)?With emphasis on biological populationsWith emphasis on biological populationsx1 09 Experimental Design 2Reading AssignmentReading AssignmentReread section 1.5 Reread section 1.5 ““Doing BiologyDoing Biology””p. 8p. 8--12.12.Figure 35.9 (p.681) presents a type of Figure 35.9 (p.681) presents a type of experiment that may not evaluated statistically experiment that may not evaluated statistically because outcomes are + or because outcomes are + or --..x1 09 Experimental Design 3Types of ScienceTypes of ScienceDiscoveryDiscoveryLook for (or try) something new (not known)Look for (or try) something new (not known)MeasurementMeasurementInstrumentation for more precision or accuracyInstrumentation for more precision or accuracyExperiment (hypothesis testing)Experiment (hypothesis testing)Comparing groups, interpreting outcomesComparing groups, interpreting outcomesTheory, logic, mathematicsTheory, logic, mathematicsModels of global climateModels of global climatex1 09 Experimental Design 4Practice versus sciencePractice versus scienceDoctors and engineers use the knowledge Doctors and engineers use the knowledge generated by science to fix or build things.generated by science to fix or build things.This is also a creative process as the most This is also a creative process as the most knowledge (generalities) have to be applied to the knowledge (generalities) have to be applied to the particulars of individuals or circumstances.particulars of individuals or circumstances.Science focuses on building our knowledge Science focuses on building our knowledge base and does not emphasize the expected base and does not emphasize the expected usefulness of knowledge.usefulness of knowledge.x1 09 Experimental Design 5GeneralityGeneralityScience seeks knowledge that applies to all Science seeks knowledge that applies to all individuals of a individuals of a ‘‘typetype’’..All Carbon atoms behave in a certain way, but All Carbon atoms behave in a certain way, but more refined measurements may be able to more refined measurements may be able to distinguish distinguish 1212C from C from 1313C, then the two isotopes C, then the two isotopes can be subpopulations of C.can be subpopulations of C.In biology populations within a population are In biology populations within a population are often distinguished by gender or age.often distinguished by gender or age.x1 09 Experimental Design 6Constants, Variables, ParametersConstants, Variables, ParametersConstants donConstants don’’t ever change, e, t ever change, e, ππ, 5, 5..The word variable is used for attributes that do The word variable is used for attributes that do change, either thru time or among individuals in a change, either thru time or among individuals in a population. population. Some variable take only two values (states), e.g., Some variable take only two values (states), e.g., [YES, NO], others can take any value.[YES, NO], others can take any value.A parameter is a value that does not change within A parameter is a value that does not change within the equation used, but changes when the equation is the equation used, but changes when the equation is applied to different types, i.e., half lives or doubling applied to different types, i.e., half lives or doubling times.times.x1 09 Experimental Design 7Types of VariablesTypes of VariablesMeasurement variablesMeasurement variablesIndividuals are measured on a continuous, ordered Individuals are measured on a continuous, ordered scale (all the attributes we talked about in lecture 2 scale (all the attributes we talked about in lecture 2 ––length, mass, speed, etc.length, mass, speed, etc.Median, median, variance, standard deviation Median, median, variance, standard deviation apply to measurement variables.apply to measurement variables.Categorical variablesCategorical variablesThere are a small number of types and individuals There are a small number of types and individuals are sorted into types and then counted, e.g., red are sorted into types and then counted, e.g., red eyes or white eyes.eyes or white eyes.x1 09 Experimental Design 8PopulationsPopulationsIndividuals within a population are variable with Individuals within a population are variable with respect to many attributes.respect to many attributes.Often it is useful to divide a population into Often it is useful to divide a population into subpopulations. For example, humans may be subpopulations. For example, humans may be better understood by dividing the whole into better understood by dividing the whole into groups based on age.groups based on age.We prefer to make statements (hypotheses) We prefer to make statements (hypotheses) about more inclusive subpopulations.about more inclusive subpopulations.x1 09 Experimental Design 9StatisticsStatisticsAny value that is calculated from a set of Any value that is calculated from a set of observations is a statistic. A statistic conveys observations is a statistic. A statistic conveys information about the observed values without giving information about the observed values without giving all the observations. all the observations. Prevalence and Incidence (introduced in lecture 8) are Prevalence and Incidence (introduced in lecture 8) are statistics of a disease in a population.statistics of a disease in a population.Mean and median are statistics that measure Mean and median are statistics that measure ‘‘central central tendencytendency’’--where middle is.where middle is.x1 09 Experimental Design 10Causation & CorrelationCausation & CorrelationIf we apply an action to individuals