Principles of exp. design Control for effects of lurking

variables Randomization to keep personal

biases or other preferences out of the study

Replication of the experiment reduces chance variation

Completely randomized design

All experimental units are allocated at random among all treatments.

Ex: Clinical trial to test effectiveness of new drug vs. current standard for alleviating migraine headaches. Test subjects are people known to have migraine headaches.

More complicated designs In some cases, we can use the idea

of randomization to develop more complex and more efficient designs

There are many special cases – we’ll touch on matched pairs and block designs

Matched pairs designs We can sometimes match an

experimental unit of the control group with one from the treatment group (can extend to more groups)

Ex: Studies with identical twins as volunteers – can randomly assign one to each treatment (or control)

Block designs Similar experimental units assigned to

same block Blocks differ in ways which may affect

experiment Ex: In agricultural field trials, each plot of

similar soil type may form block Random assignment of treatments made

within each block

Probability/inference

Population Sample

Use inference

Use probability

Population vs. sample Population is entire group we want

to get information about Sample is subset of population that

we are able to survey or examine Make inference about population by

looking only at sample Sample design is method used to

choose sample

Voluntary response sample Data/responses from people who

respond to a general appeal/request Tends to be biased, because

respondents are mostly those who have time to respond or are motivated by strong opinions

Not necessarily indicative of whole population

Survey problems Undercoverage: some groups in the

population are left out of the process of choosing the sample.

Nonresponse: individual chosen for the sample can’t be contacted or does not cooperate.

Response bias: behavior of interviewer or respondent can influence responses. Wording of questions Desire of respondent to “look good”

Simple random sample Is special case of probability design,

which gives each individual some chance of being chosen

Sample size usually denoted by n Sample chosen from population so that

each subset of size n has same chance of being selected

Since each individual has same chance of being chosen, there’s no systematic bias

Stratified random sample By chance, a simple random sample may

not get enough people from important groups.

Solution: take a simple random sample of each group (stratum) separately

Combine simple random samples to obtain a stratified random sample (similar to blocking)

Multistage samples Hard to get a simple random sample on a

large scale (how to choose, how to physically get there for interview, etc.)

Solution: choose sample in stages Randomly choose areas (for instance,

counties), then further subdivide (townships)

Subdivide into neighborhoods or other small areas

Take sample of houses in these areas

Survey problems Undercoverage: some groups in the

population are left out of the process of choosing the sample.

Nonresponse: individual chosen for the sample can’t be contacted or does not cooperate.

Response bias: behavior of interviewer or respondent can influence responses. Desire of respondent to “look good” Wording of questions

Dealing with non-response Persistence

If at first you don’t succeed… Methodology

Which methods below do you think will have the biggest problem with non-response?• Telephone interview• Mailings• Email survey• Face-to-face interview

Some cell phone users have developed brain cancer. Should all cell phones come with a warning label explaining the danger of using cell phones?

Do you agree that a national system of health insurance should be favored because it would provide health insurance for everyone and would reduce administrative costs?

In view of escalating environmental degradation and incipient resource depletion, would you favor economic incentives for recycling of resource-intensive consumer goods?

Which of the following best represents your opinion on gun control? The government should take away our

guns. We have the right to keep and bear

arms.