More DesignsSection 4.2B

BlockGroup of experimental units or

subjects that are known beforethe experiment to be similar insome way that is expected to

systematically affect the response to the treatments.

*Similar to stratified samples in surveys.

Randomized Block DesignRandom 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!

Subjects

Men

Women

Random assignment

Random assignment

Group 1

Group 3

Group 2

Group 1

Group 2

Group 3

Therapy 1

Therapy 2

Therapy 3

Therapy 1

Therapy 2

Therapy 3

Compare survival

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