Silent Do Now (5 minutes) *Before you begin, grab a new weekly sheet and take out your homework!  An opinion poll calls 2000 randomly chosen residential telephone numbers then asks to speak with an adult member of the household. The interviewer asks, “how many movies have you watched in a movie theatre in the last 12 months?” In all, 1131 people respond.  Identify a potential source of bias related to the question being asked. Suggest a change that would fix the problem.  Identify another potential source of bias in this survey that is unrelated to the once you just mentioned. Suggest a change that would help fix this problem.

Important Definitions  Observational Study Vs. Experiment  Tell it in your own words!

Important Definitions  Experimental Units - Individuals on which the experiment is being done  Subjects – what we call experimental units that are human  Factors – explanatory variables in an experiment  Levels – created through the joint effects of several factors  Response Variable – The output of your experiment  Treatment – The experimental condition placed on the experimental units

Important Definitions  Control Group–  The group that we compare to the treatment group.  The group that receives either no treatment or a standard treatment.

Important Definitions  Placebo Effect – When patients respond favorably to a dummy treatment (a placebo)  The response can be due to trust in the doctor and expectations of a cure or simply to the fact that conditions often improve without treatment

Important Definitions  There are two main classifications of people who can affect the outcome of an experiment.  Those who could influence the results (subjects, administrators of treatments, etc)  Those who evaluate the results  Blind – Either the subjects or those who measure the response variable (evaluators of the experiment) are unaware of which group the subject has been assigned to.  Double Blind – Neither the subjects nor those who measure the response variable (evaluators of the experiment) know which treatment a subject received

Comparative Experiments  In order to measure the causal effect of a treatment, it is desirable to have more than one group to compare results.  This may mean we are comparing two or more levels of a factor.  It may also mean that we are comparing a single level of a factor to a group which does not receive the treatment.

Four Principles of Experimental Design 1.Control 2.Randomization 3.Replication 4.Blocking The first three MUST be present in every experiment in order to apply it to the general population, the last is not always necessary.

Control  Sources of variation in a variable can come from many places  We must identify any possible sources of variation besides the treatment itself and then control these sources as best we can. For Example: If I am trying to see how rapidly bacteria will grow on a petri dish in a laboratory, I must control the environment of the laboratory to ensure that chemicals or other substances don’t affect the bacteria.

Randomization  Randomization is just as important in designing an experiment as it is in selecting a sample.  Randomizing which individuals get which treatments helps us to spread the variation between individuals out across the groups.  Randomizing is a way of maintaining control over sources of variation.

Replication  There are two kinds of replications in comparative experiments  Applying the treatments to more than one individual  Replicating the entire study  This allows us to generalize to the larger population with more confidence.

Blocking  Blocking, like stratification in sampling, allows us to identify a variable we know will have different results and thus control this variable  To block we:  Place all individuals in one group by the identified variable  We use randomization on the resulting groups to all levels of the treatment. For Example: If we block by gender, because we believe that we will receive different results for each gender, we essentially are running separate experiments on each gender and analyzing the results separately.

Types of Experimental Designs  Completely Randomized Design – All experimental units are allocated at random among all treatments.  Example:

Types of Experimental Designs  Randomized Block Design – Random assignment of units to treatments is carried out separately within each block.  Example:

Types of Experimental Designs  Matched Pairs Design – Compares only two treatments.  Subjects are matched in pairs For Example: An experiment to compare two advertisements for the same product might use pairs of subjects with the same age, sex and income. The idea is that matched subjects are more similar than unmatched subjects, so comparing responses within a number of pairs is more efficient than comparing responses of groups of randomly assigned subjects.

Example of Matched Pairs Design

Repeated Measures Design  A repeated measures design is a form of matched pair design where each subject serves as their own control.  In other words, every subject will take both the new medication and the old medication and our response variable will again be the difference in pain relief for each subject.

How to Design an Experiment 1.Identify the important FEATURES of the experiment. 2.Draw a DIAGRAM for the randomized design. 3.Explain how the design utilizes the PRINCIPLES OF EXPERIMENTAL DESIGN.

Example 1:  A farm-product manufacturer wants to determine if the yield of a crop is different when the soil is treated with three different types of fertilizers. Fifteen similar plots of land are planted with the same type of seed but are fertilized differently. At the end of the growing season, the mean yield from the sample plots is compared.  Identification of Features:  What are the experimental units?  What is the factor (explanatory variable)?  How many levels?  What is the response variable?  How many treatments?

 Diagram:  Four Principles of Design:

Quick Check!

Example 2: Randomized Block Design  A pharmaceutical company has developed a new medicine that they believe will be more effective in treating people who suffer from migraine headaches. Researchers plan to enlist several people who suffer from migraines in a test.  Identification of Features:  What are the experimental units?  What is the factor (explanatory variable)?  How many levels?  What is the response variable?  How many treatments?

 Diagram:  Four Principles of Design:

Quick Check! (Pg. 374 #5.5 in textbook!)  Oops!!!

Help! Help! Help! HOW DO WE STAY ETHICAL YET ELIMINATE BIAS?

Informed Consent  A subject should have know that they are a part of an experiment, and that the possibility exists for them to be assigned to one of several groups. This way the subject can either choose to participate or not. What they won’t be told is which group they will be assigned.

Independent Practice  Finish for homework (read carefully!!!)  Pg & 5.53