By Sahar Abdullah and Jimin Kim Vocabulary Observational study a study based on data in which no manipulation of factors has been employed Retrospective study an observational study in which subjects are selected and then their previous conditions or behaviors are determined ID: 562072
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Slide1
Chapter 13: Experiments and Observational Studies
By: Sahar Abdullah and Jimin Kim Slide2
Vocabulary
Observational study:
a study based on data in which no manipulation of factors has been employed
Retrospective study:
an observational study in which subjects are selected and then their previous conditions or behaviors are determined.
Prospective study:
an observational study in which subjects are followed to observe future outcomes
Experiment:
manipulates factors levels to create treatments, randomly assigns subjects to treatment levels, and then compares the responses of the subject groups across treatment levels
Random assignment:
assigning experimental units to treatment groups at random. Slide3
Vocabulary
Factor:
a variable whose
levels
are manipulated by the experimenter
Response
:
a variable whose values are compared across different treatments
Experimental units:
individuals on whom experiment is performed. Called subjects or participates when the individual is a human
Level:
the specific values that the experimenter chooses for a factor
Treatment:
the process, intervention, or other controlled circumstance applied to randomly assigned experimental units; the different levels of a single factors or a combination of levels of two or more factorsSlide4
Vocabulary
Principles of experimental design
Control
Randomize
Replicate
Block
Statistically significant:
observed difference that is too large
to
believe that they are a result
of
random fluc
tuation
Control group:
this group
receives the baseline treatment level; responses provide a basis for comparisonSlide5
Vocabulary
Blinding:
Keep individuals from knowing how the subjects are assigned to treatment groups; helps eliminate bias
Single blind:
when
those who can influence the results (ex. subjects) are blind
Double blind:
when
those who can influence the results (ex. subjects) and those who can evaluate the results (ex. treating physicians) are both blind
Placebo:
a tre
atment known to have no effect; administered so that all groups experience the same conditions
Placebo effect:
the tendency of many human subjects to show a re
sponse even when administered a placeboSlide6
Vocabulary
Blocking:
isolating the
variability
attributed to the differenced
Matching:
reduces unwanted variation
Design
s:
in a randomized block design, the randomization occurs only within blocks
Confounding:
when
the levels of one factor are associated with the levels of another factor so their effects cannot be separatedSlide7
Main concept
- Principles of Experimental Design(“Control what you can and randomize the rest.”)
Control
sources of variation that are not being studied, but will have an impact on the results. This makes conditions as similar as possible for all treatment groups.
Note: the results of the study cannot be generalized (i.e., extrapolated) to other levels of the controlled variables.
Replicate
over as many experimental units as possible. Just as for simulations, replication reduces random variability in an experiment.
Randomize
experimental units to treatments. This equalizes the effects of unknown or uncontrollable sources of variation.
Block
to reduce the effects of things that cannot be controlled. Blocking is like stratifying a sample; we separate dissimilar experimental units into separate blocks, then randomize by treatment within each block. In effect, the experiment is run separately on each block; the results are combined across blocks for purposes of study. Slide8
Elements of an Experiment
Plan:
State what you want to know.
Response:
Specify the response variable.

Treatments:
Specify the factor levels and treatments.
Experimental Units:
Specify the experimental units.
Experimental Design:
Control, replicate and randomly assign (also, block if it helps).
Draw a Diagram
Perform the Experiment
Describe the Results:
And, determine whether the results are statistically significantSlide9
Homework Problems
35. Some schools teach reading using phonics (the sounds made by letters) and others using whole languages (word recognition). Suppose a school district wants to know which method works better. Suggest a design for an appropriate experiment.
Treatment:
This experiment has 1 factor (reading program), at 2 levels (phonics and whole language), resulting in 2 treatments.
Response variable:
the reading score on an appropriate reading test after a year in the program.
Design:
Randomly assign half of the teachers to use phonics and the other half to use whole language. Then randomly assign students to teachers. There may be variation in reading score in different schools within the district, as well as by grade. Blocking by both school and grade will reduce this variation.Slide10
Homework Problems
37. A study published in the
New England Journal of Medicine
(Aug. 2001) suggests that it’s dangerous to enter a hospital on a weekend. During a 10-year period, researchers tracked over 4 million emergency admissions to hospitals in Ontario, Canada. Their findings revealed that patients admitted on weekends had a much higher risk of death than those who went to the emergency room on weekdays.
a) The researchers said the difference in death rates was “statistically significant.” Explain in this context what this means.
The difference between the death rate on the weekend and the death rate during the week is greater than would be expected due to natural sampling variation.
b) What kind of study was this. Explain.
It is a prospective observational study. The researchers identifies hospitals in Ontario, Canada, and tracked admissions to the emergency rooms. This cannot be an experiment since people cannot be assigned to become injured on a specific day of the week.Slide11
Homework Problems
37. A study published in the
New England Journal of Medicine
(Aug. 2001) suggests that it’s dangerous to enter a hospital on a weekend. During a 10-year period, researchers tracked over 4 million emergency admissions to hospitals in Ontario, Canada. Their findings revealed that patients admitted on weekends had a much higher risk of death than those who went to the emergency room on weekdays.
c) If you think you’re quite ill on Saturday, should you wait until Monday to seek medical help? Explain.
No, it would be foolish to wait since it is likely that confounding variables accounted for the higher death rate on the weekends. For example, people may have participated in riskier activities on the weekend.
d) Suggest some possible explanations for this troubling finding.
Drinking may have lead to a higher death rate. Perhaps, more people drank alcohol, which may have lead to more traffic accidents, and higher rated of violence.