More Designs
Section 4.2B
Block
Group of experimental units or
subjects that are known before
the experiment to be similar in
some way that is expected to
systematically affect the response
to the treatments.
*Similar to stratified samples in
surveys.
Randomized Block Design
Random assignment of units to treatments is carried out separately
within each block.
Example
The progress of a type of cancer differs in women & men. We
want to test 3 therapies for this cancer – thus we block for
gender.
Let’s try!
Men
Random
assignment
Group 1
Therapy 1
Group 2
Therapy 2
Group 3
Therapy 3
Compare
survival
Subjects
Group 1
Women
Random
assignment
Group 2
Group 3
Therapy 1
Therapy 2
Therapy 3
Compare
survival
Form blocks on the most important
unavoidable sources of variability
among the experimental units.
Control what you can, block what you can’t control, and randomized
the rest.
Block into Homogenous Groups.
Matched Pairs Design
• Compares just two treatments
• Subjects are matched in pairs
• There’s still randomization – which treatment they get
• Can use the same subject for both treatments.
Example
Cell phone & driving experiment
All subjects drove both with and without using the cell phone.
Researchers compared each individual’s reaction times with
and without the phone.
If all subjects drove 1st with the phone and then without it – the
effect of talking on the cell phone would be confounded with the
fact that it was the first run in the simulator.
Thus we randomize the order.
Let’s Review the 3 designs:
Don’t forget to be very specific
when describing how you
randomize!
Classwork
• Blocking Worksheet
Homework
• Worksheet