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Chapter 13

# STAB22H3 Chapter Notes - Chapter 13: Random Assignment, Design Of Experiments, Menopause

Department
Statistics
Course Code
STAB22H3
Professor
Ken Butler
Chapter
13

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CHAPTER 13 – EXPERIMENTS AND OBSERVATIONAL STUDIES
WHERE ARE WE GOING?
4 principles of experimental design
1. control
2. randomize
3. replicate
4. block if appropriate
together these 4 principles depict the ideal experiment & how to understand
results that come from it
much of knowledge that is known about social sciences, & natural sciences
derive from carefully designed expts
MAIN BODY
Example (Music)
study wants to look at who gets better grade: students who play musical
instruments or those who do not
found that, by comparing the performance of those who play and those who
do not, the ones that play have higher overall GPA
3.59 vs. 2.91
16% of music students had As, while only 5% for non-music
OBSERVATIONAL STUDIES
[1]
Example (Music)
study attempted to show assoc. b/ween music edu. & grades
simply observed students in their envirnmt, recorded the choices they made
(music or non-music) and then outcome (GPA)
OBSERVATIONAL STUDIES
- ex. is Example (Music)
- no assigning of choices is done
- merely observing
RETROSPECTIVE STUDY
- observational study that involves
- identifying subj's from records
- looking at subjs' past results
- Example (Music) - collected data about students' past grades, and saw whether
or not they played music
[2]
Why can we not conclude that music education CAUSES good grades?
- for music education to cause good grades, this requires that there are no other

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diff's b/ween the two groups being looked at that can account for the diff's in the
- for this Ex., cannot conclude causation b/c whether or not they studied music
was NOT the only diff. b/ween the 2 groups
[3]
Consider lurking variables - Example (Music)
- var's that are we have not taken into acc't may have caused groups to perform
diffntly
- ex. work ethic
- ex. parental support
- ex. wealth
- ex. level of intelligence (prior to music education)
^- these are ex's of var's that could have caused diff's b/ween the performances
of the two groups
[4]
RETROSPECTIVE STUDIES
- restrospective records can potentially have errors, b/c its based on historical
data
- often are used to discover var's related to rare outcomes (Ex. specific diesases)
- first identify ppl w/ disease
- then look into history & heritage to find things related to this diseased
condition
- have restricted view of world b/c typically are restricted to small part of popn
- ex. identifying ppl w/ that specific disease; only minority may even have it
in the first plac e
[5]
PROSPECTIVE STUDIES
- observational study that involves
- collecting data as events occur
- observes subj's over time
- record var's of interest, and see what is outcome as it occurs
- Example - Music
- select students who are going to join music lessons but have not do so yet
- track their academic performance over several yrs
- compare these music students' results to another group of subj's that we
track which aren't taking music lessons

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[6]
OBSERVATIONAL STUDIES
- CANNOT POSSIBLY DETERMINE CAUSATION
- regardless of if PROSPECTIVE, or RETROSPECTIVE
- doesn't guarantee that impt, related var's to the outcome are the most impt,
or even CORRECT
- Example - Music
- non-music vs. music might still differ in an IMPT way that was failed to be
observed
- could be this unobserved var. that caused good grades, instead of music
per se
RANDOMIZED, COMPARATIVE EXPERIMENTS
[1]
EXPERIMENT
- Example - Music
- take a group of students
- randomly assign half to take music lessons, and the other half never
- then, compare their grades several yrs after
[2]
EXPERIMENT
- REQ. random assignment of subj's to treatments
- Examples of questions that a designed experiment can justify
- does music lessons cause good grades?
- does taking Vitamin C decrease the chance of getting a cold?
- does working with computers improves performance in STAB22?
- is this drug a safe, health-conscious treatment for this disease?
[3]
EXPERIMENT
- studies relationship b/ween 2/more var's
- must identify at least 1:
- FACTOR - name for EXPLANATORY VARIABLE in expts
- RESPONSE VARIABLE
How does experiment differ from other forms of investigation?
- expter..
- actively & intentionally manipulates factors to control details of possible
treatments
- randomly assigns subj's to treatments