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!

Subjects Men Women 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

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 1 st 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