chapter 13 notes observational studies and experimental design observational studies in...
TRANSCRIPT
Chapter 13 NotesObservational Studies and
Experimental Design
Observational Studies
In observational studies, researchers don’t assign choices; they simply observe choices and their results. Observational studies can be retrospective (study things that happened in the past) or prospective (identify subjects in advance and collect data as events unfold.).
Chapter 13 NotesObservational Studies and
Experimental Design
Observational Studies
Observational studies are valuable for discovering trends and possible relationships. Some things can only be studied through observational studies because an experiment is either unethical or impossible to achieve.
Chapter 13 NotesObservational Studies and
Experimental Design
Observational Studies
Although observational studies may identify important variables related to outcomes we are interested in, it is not possible for them to demonstrate a causal (cause & effect) relationship.
Chapter 13 NotesObservational Studies and
Experimental Design
Experiment:
An experiment requires a random assignment of subjects to treatments.
Only an experiment can justify a cause & effect relationship
Chapter 13 NotesObservational Studies and
Experimental Design
Experiments
• An experiment must identify at least one explanatory variable, called a factor, to manipulate and at least one response variable to measure
Chapter 13 NotesObservational Studies and
Experimental Design
Experiments
• The experimenter actively and deliberately manipulates the factors to control the details of the possible treatments, and assigns the subjects to those treatments at random.
Chapter 13 NotesObservational Studies and
Experimental Design
Experiments
• Humans who are experimented on are called subjects or participants while other individuals (plants, rats, bacteria, etc…) are called experimental units.
Chapter 13 NotesObservational Studies and
Experimental Design
Experiments
• The specific values that the experimenter chooses for a factor are called the levels of the factor.
Chapter 13 NotesObservational Studies and
Experimental DesignExperiments
• The combination of all factors at all levels that an experimental unit might receive are known as the treatments.
Chapter 13 NotesObservational Studies and
Experimental Design
Four Principles of Experimental Design
• Control – We control sources of variation other than the factors we are testing by making conditions as similar as possible for all treatment groups.
Chapter 13 NotesObservational Studies and
Experimental Design
Four Principles of Experimental Design
•Randomize – randomization allows us to equalize the effects of unknown or uncontrollable sources of variation. It reduces bias.
Chapter 13 NotesObservational Studies and
Experimental DesignFour Principles of Experimental Design
•Replicate – We repeat the experiment, applying the treatments to a number of subjects. We also replicate within different sampling frames to be sure our population of interest is represented.
Chapter 13 NotesObservational Studies and
Experimental DesignFour Principles of Experimental DesignBlocking •This is done homogeneously.•The motivation is because you believe the characteristic you choose to block on will effect the outcome or response variable. •A matched pair design is a special sort of blocking. We pair similar items and compare the pairs.
Chapter 13 NotesObservational Studies and
Experimental Design
If we observe results in an experiment that we believe could not be attributed to chance, then we say the results are statistically significant.
Chapter 13 NotesObservational Studies and
Experimental Design• Randomization in sampling and experiments are
different and are done for different reasons. • Samples are randomized to be sure that the
population of interest is represented- unbiased.• Experimental units or subjects are not randomly
selected from the population but are volunteers/participants that are randomly assigned to each treatment group.
Chapter 13 NotesObservational Studies and
Experimental Design• A comparative experiment will often use a
control group. We must have something to compare our new treatment against and we often create a control group with a control treatment. This control group is typically a treatment involving a former method or a placebo.
Chapter 13 NotesObservational Studies and
Experimental Design
• A “fake” treatment that looks like the treatment being tested is called a placebo. The Placebo Effect is a reaction to a placebo based on what the patient “believes” he/she is taking.
Chapter 13 NotesObservational Studies and
Experimental Design
• When it can’t see the results of one factor because another factor is effecting the experiment too, we say that these two factors are confounded.
• PLEASE read p. 308 – the section on Lurking or Confounding?
Chapter 13 NotesObservational Studies and
Experimental Design
Blinding
• Blinding refers to not knowing what treatment group a participant is a part of.
Chapter 13 NotesObservational Studies and
Experimental Design
Blinding • There are two main classes of individuals who
can affect the outcome of an experiment:• **those who could influence the results (the
subjects, treatment administrators, or technicians)
• **those who evaluate the results (judges, treating physicians, etc…)
Chapter 13 NotesObservational Studies and
Experimental DesignBlinding• When all the individuals in either one of
these classes is blinded, an experiment is said to be single-blind. When everyone in both classes is blinded, we call the experiment double-blind. (It is true however that someone must know what group people are in, just not any of the people in these classes.)