1. Chapter 13 Notes Observational Studies and Experimental Design
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.).

2. Chapter 13 Notes Observational Studies and Experimental Design
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.

3. Chapter 13 Notes Observational Studies and Experimental Design
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.

4. Chapter 13 Notes Observational Studies and Experimental Design
An experiment requires a random assignment of subjects to treatments.
Only an experiment can justify a cause & effect relationship

5. Chapter 13 Notes Observational 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

6. Chapter 13 Notes Observational 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.

7. Chapter 13 Notes Observational 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.

8. Chapter 13 Notes Observational Studies and Experimental Design
Experiments
The specific values that the experimenter chooses for a factor are called the levels of the factor.

9. Chapter 13 Notes Observational Studies and Experimental Design
Experiments
The combination of all factors at all levels that an experimental unit might receive are known as the treatments.

10. Chapter 13 Notes Observational 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.

11. Chapter 13 Notes Observational 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.

12. Chapter 13 Notes Observational Studies and Experimental Design
Four 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.

13. Chapter 13 Notes Observational Studies and Experimental Design
Four Principles of Experimental Design
Blocking
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.

14. Chapter 13 Notes Observational 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.

15. Chapter 13 Notes Observational 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.

16. Chapter 13 Notes Observational 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.

17. Chapter 13 Notes Observational 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.

18. Chapter 13 Notes Observational 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?

19. Chapter 13 Notes Observational Studies and Experimental Design
Blinding
Blinding refers to not knowing what treatment group a participant is a part of.

20. Chapter 13 Notes Observational 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…)

21. Chapter 13 Notes Observational Studies and Experimental Design
Blinding
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.)