Designing ExperimentsPart II

Section 5.2

Block Design

A block is a 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.

Blocks are a form of control. In a block design, the random

assignment of units to treatments is carried out separately within each block.

Example

Suppose a fitness instructor believes that a certain exercise regimen will increase upper-body strength. He recruits students to test his theory by having them do as many push-ups as they can after they complete the training.

What might be a cause for variability in the results?

What could be done to account for this variability?

Outline of a block design for comparing cancer therapies.

The blocks consist of male and female subjects. The treatments are three therapies for cancer.

Remember!!!!

A wise experimenter will form blocks on the most important unavoidable sources of variability among experimental units.

Control what you can,Block what you can’t,And randomize the rest.

Matched Pairs Designs

Matching subjects in different ways can produce more precise results than simple randomization.

The simplest use of matching is a matched pairs design, which compares just two treatments.

The subjects are matched in pairs with similar subjects and one receives the treatment and one does not.

Cell phones and driving 2Matched pairs

In the cell phone study, two treatments were studied. Driving in a simulator and driving a simulator while talking on a cell phone. The braking times were compared.

In a matched pairs design, all students drove with and without using the cell phone.

The order in which they drove with and without the cell phone was random to reduce the effect of it being the first time they drove the simulator.

Double-Blind Experiment

In a double-blind experiment, neither the subjects nor those who measure the response variable know which treatment a subject received.

The double-blind method avoids unconscious bias, by for example, a doctor who doesn’t think that “just a placebo” can benefit a patient.

Lack of realism

Placebo cigarettes?

A study of the effects of marijuana recruited young men who used marijuana. Some were given marijuana while others were given placebo cigarettes. The one with the placebos knew they were not real and complained loudly.

Lack of realism

Response to advertising

A study compares two TV ads by showing TV programs to student subjects. The students know it is “just an experiment”.

We can’t be sure that the results apply to everyday television viewers.