Understanding Experiments Robb T Koether Homework Review Introduction Control Groups Understanding Experiments Lecture 10 Section 3 5 Randomized Design Blinded Experiments Assignment Robb T Koether Hampden Sydney College Fri Jan 30 2008 Outline Understanding Experiments Robb T Koether Homework Review 1 Homework Review 2 Introduction 3 Control Groups 4 Randomized Design 5 Blinded Experiments 6 Assignment Introduction Control Groups Randomized Design Blinded Experiments Assignment Homework Review Understanding Experiments Robb T Koether Exercise 2 32 p 122 Homework Review Introduction Control Groups Randomized Design Blinded Experiments Assignment A 1 in 14 systematic sample will be taken from a population of 555 members of an art appreciation club The 555 members are labeled from 1 through 555 a What is the chance that the member with label 17 will be selected b Suppose the sample is selected and the number with label 17 is included What is the sample size for the resulting sample Homework Review Understanding Experiments Robb T Koether Homework Review Introduction Control Groups Randomized Design Blinded Experiments Assignment Solution a Because we are using 1 in 14 sampling the chance is 1 14 If 17 was included then the selected random number b must have been 3 3 14 17 9 Also 555 14 39 14 That means that there are 39 full blocks of size 14 and one partial block of size 9 Since 3 9 we will pick up one member from the partial block That will make the sample size 40 Introduction Understanding Experiments Robb T Koether Homework Review Introduction Control Groups Randomized Design Blinded Experiments Assignment Suppose a drug is given to 100 patients suffering from a particular disease After 2 weeks 90 of the patients have recovered The researchers conclude that the drug was effective What is wrong with this Introduction Understanding Experiments Robb T Koether Homework Review Introduction Control Groups Randomized Design Blinded Experiments Assignment Suppose a drug is given to 100 patients suffering from a particular disease After 2 weeks 90 of the patients have recovered The researchers conclude that the drug was effective What is wrong with this Introduction Understanding Experiments Robb T Koether Homework Review Introduction Control Groups Randomized Design Blinded Experiments Assignment Suppose a drug is given to 100 patients suffering from a particular disease After 2 weeks 90 of the patients have recovered The researchers conclude that the drug was effective What is wrong with this Introduction Understanding Experiments Robb T Koether Homework Review Introduction Control Groups Randomized Design Blinded Experiments Assignment Suppose a drug is given to 100 patients suffering from a particular disease After 2 weeks 90 of the patients have recovered The researchers conclude that the drug was effective What is wrong with this Treatment and Control Groups Understanding Experiments Robb T Koether Homework Review Definition Treatment group Introduction The treatment group is the group that receives the treatment Control Groups Randomized Design Blinded Experiments Assignment Definition Control group The control group is similar to the treatment group in all respects except that it does not receive the treatment Randomized Design Understanding Experiments Robb T Koether Homework Review Introduction Control Groups Randomized Design Blinded Experiments Assignment Why would it be wrong to allow the individuals themselves to choose whether to be in the treatment group or the control group Why would it be wrong for the researchers to decide subject by subject who goes into which group Randomized Design Understanding Experiments Robb T Koether Homework Review Introduction Control Groups Randomized Design Blinded Experiments Assignment Definition Randomized design A randomized design is a design in which the subjects are randomly assigned to either the treatment group or the control group Randomized Design Understanding Experiments Robb T Koether Homework Review Introduction Control Groups Randomized Design Blinded Experiments Assignment Randomized Design Suppose that there are 100 subjects Number them 1 100 Then use a random number generator to obtain 50 distinct random numbers from 1 100 Those 50 subjects would be assigned to the treatment group The rest would be assigned to the control group Possible Bias Understanding Experiments Robb T Koether Homework Review Introduction Control Groups Randomized Design Blinded Experiments Assignment Are the subjects in the treatment group aware of the purpose of the experiment Are the subjects in the control group aware that they are not receiving the drug Will it make a difference Two Types of Bias Understanding Experiments Robb T Koether Homework Review Introduction Control Groups Definition Response bias A sampling method exhibits response bias if the subjects give what they perceive to be the desired response rather than the true response Randomized Design Definition Experimenter bias Blinded Experiments A sampling method exhibits experimenter bias if the observer records the desired values rather than the true observed values Assignment How can we avoid these biases Placebos Understanding Experiments Robb T Koether Homework Review Introduction Control Groups Definition Placebo A placebo is a treatment usually a pill that is known to have no effect Randomized Design Definition Single blind experiment Blinded Experiments A single blind experiment is an experiment in which the subjects do not know who is in the treatment group and who is in the control group Assignment Placebos Understanding Experiments Robb T Koether Homework Review Introduction Control Groups Randomized Design Blinded Experiments Assignment Everybody in the treatment group is administered the drug Everybody in the control group gets the placebo The researchers look for differences in the groups recovery rates Double Blind Experiments Understanding Experiments Robb T Koether Homework Review Introduction Control Groups Randomized Design Blinded Experiments Assignment Definition Double blind experiment A double blind experiment is an experiment in which neither the subjects nor the observers know who is in the treatment group and who is in the control group Assignment Understanding Experiments Robb T Koether Homework Review Introduction Control Groups Randomized Design Blinded Experiments Assignment Homework Read Section 3 5 pages 168 171 173 182 Let s Do It 3 5 3 6 3 7 3 8
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