AP Statistics
Ch.4.2: Completely Randomized Design
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From your Chapter 4 Packet, 4.2 Concept #2:
1. What are the experimental units, explanatory
variable, treatments and response variable?
Experimental units: Each of the 150 students
Explanatory variable: classical music
Treatments: classical music, no music
Response variable: test scores
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2. What are potential lurking variables in this
situation?
Study time, tutoring, amount of sleep, etc.
How could they affect the results?
More studying, tutoring, and more sleep could lead
to higher test scores
How could we avoid their effects?
Randomly assign students to groups so that the
effects of lurking variables will be felt equally by all
groups in the experiment.
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3. Describe a completely randomized design for
Mr. Tyson’s experiment.
Group 1
75
students
150 statistics
students
Treatment 1
classical
music
Compare
Test scores
Random
Assignment
Group 2
75
students
Treatment 2
No music
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BONUS: How would you carry out the random assignment?
Provide enough detail that a classmate could implement your
procedure exactly as written.
Random number method: Using an alphabetical list of the
students, assign each student a number between 01 and 150.
Use a calculator to randomly generate 75 unique numbers
between 01 and 150. These students belong in the treatment
group, where students will listen to classical music while
studying. The other students will listen to no music.
Hat method: Use 150 equally sized slips of paper. Label 75 of
the slips “1” and 75 of the slips “2.” Then mix them up in a hat
and have each student draw a number without looking. The
students who draw “1” will be assigned to the group that
listens to classical music while studying and the students who
draw “2” will be assigned to the group that does not listen to
music while studying.
Quick Quiz Solutions
1. What are the experimental units, explanatory
variable, treatments and response variable?
Experimental units: Each of the 29 students
Explanatory variable: small group discussions
Treatments: small group discussions, view video
alone (no small group discussion)
Response variable: performance evaluation
Quick Quiz Solutions
2. How can you avoid the effect of potential
lurking variables in this experiment?
Randomly assign students to treatment groups
so that the effects of lurking variables will be felt
equally by all groups in the experiment.
Quick Quiz Solutions
3. Outline a completely randomized design for
this experiment using a flowchart.
Group 1
15
students
29 music
students
Treatment 1
small group
Compare
Evaluations
Random
Assignment
Group 2
14
students
Treatment 2
alone
Quick Quiz Solutions
2. Describe how you would carry out the random assignment.
Provide enough detail that a classmate could implement your
procedure exactly as written.
Random number method: Using an alphabetical list of the
students, assign each student a number between 01 and 29.
Use a calculator to randomly generate 15 unique numbers
between 01 and 29. These students belong in the treatment
group, where students will meet in small groups. The other
students will view the videos alone.
Hat method: Use 29 equally sized slips of paper. Label 15 of
the slips “1” and 14 of the slips “2.” Then mix them up in a hat
and have each student draw a number without looking. The
students who draw “1” will be assigned to the group that
views the video and discusses it in small groups and the
students who draw “2” will be assigned to the group that
views the video alone and does not discuss it with a group.