Types of Experimental Types of Experimental DesignsDesigns

EQ: How can I make a BETTER EQ: How can I make a BETTER experiment?experiment?

Observational Study: a study based on Observational Study: a study based on data in which no manipulation of data in which no manipulation of factors has been employedfactors has been employed

Experiment

Manipulates factor levels to create treatments

Randomly assigns subjects to the treatments

Compares the responses of the subjects across treatment levels

TerminologyTerminology

Experimental Units: individuals on which the experiment is done

Subjects: Human experimental units

Treatment: specific experimental condition applied to the units

Control group: Group that receives no treatment or a placebo

Placebo: A treatment known to have no effect

Blinding: Subject is unaware of what group they are in

Double Blinding: Researcher and subject are unaware of which treatment is being used.

Factor: An explanatory variable that is manipulated

Response:Variable that is measured in response to the factor

Study or Experiment?

Researchers have linked an increase in the incidence of breast cancer in Italy to dioxin released by an industrial accident in 1976. The study identified 981 women who lived near the site of the accident and were under age 40 at the time. Fifteen of the women had developed breast cancer at an unusually young average age of 45. Medical records showed that they had heightened concentrations of dioxin in their blood and that each tenfold increase in dioxin level was associated with a doubling of the risk of breast cancer.

Observational study

Study or Experiment?

Is diet or exercise effective in combating insomnia? Some believe that cutting out desserts can help alleviate the problem, while others recommend exercise. Forty volunteers suffering from insomnia agreed to participate in a month-long test. Half were randomly assigned to a special no-desserts diet; the others continued desserts as usual. Half of the people in each of these groups were randomly assigned to an exercise program, while the others did not exercise. Those who ate no desserts and engaged in exercise showed the most improvement.

Experiment

Principles of Experimental DesignPrinciples of Experimental DesignControl aspects of the experiment that we know may have an

effect on the response, but that are not the factors being studied.

Randomize to even out effects that we cannot control

Replicate over as many subjects as possible.

Analyzing ExperimentsAnalyzing ExperimentsAspirin Study

Replication:

Control:

Treatment :

Blinding:

Randomization:

Subjects/Units: 1000 male volunteers

Aspirn

Patients not know which pill they are taking

A group will take a placebo pillThe men will be randomly assigned to either the treatment group or placebo group.

Each treatment will be replicated 500 times

Factor: Aspirin Response: Number of heart attacks

Levels: Low dose and none (Placebo)

Randomized Experiment DesignRandomized Experiment DesignAspirin Study

Low dose aspirin

500 men Placebo

Compare number of heart attacks in each group

500 men

1000 volunteers

Groups TreatmentsR

andom

Ass

ignm

ent

CautionsCautionsLack of Blindness

•If the experimenter assessing the responses to treatments If the experimenter assessing the responses to treatments knows which treatment was given, he/she may bias the knows which treatment was given, he/she may bias the resultsresultsif a subject knows that they got the placebo treatment, they if a subject knows that they got the placebo treatment, they may under-report their responsemay under-report their response

Lack of realism

subjects or treatments or setting of an experiment may not subjects or treatments or setting of an experiment may not realistically duplicate the conditions we want to studyrealistically duplicate the conditions we want to study

OptiGro fertilizer claims that with their OptiGro fertilizer claims that with their product you will grow “Juicier, product you will grow “Juicier, tastier” tomatoes. You want to test tastier” tomatoes. You want to test the claim and determine if you need the claim and determine if you need the full dose for these results. How the full dose for these results. How would you design an experiment would you design an experiment involving 24 tomato plants and a involving 24 tomato plants and a panel of judges to judge their panel of judges to judge their tastiness.tastiness.

Optigro ExperimentOptigro Experiment

Replication:

Control:

Treatment and levels:

Blinding:

Randomization:

Subjects/Units: 24 plants

Fertilizer( none, ½, full)

Not needed

Using a group with no fertilizer allows us to compare the effect of the fertilizer

The treatment applied to each plant is random

Each treatment will be replicated 8 times

Factor: Fertilizer Response: Plant Growth

Completely randomized design

24 plants

8 plants

8 plants

8 plants

No fertilizer

½ dose of fertilizer

Full Fertilizer

Compare quality

Sources of variationSources of variation

What if we could only get 12 plants What if we could only get 12 plants for store A and had to buy 12 plants for store A and had to buy 12 plants from another store, store B. Should from another store, store B. Should we consider this in the experiment we consider this in the experiment design?design?

BlockingBlocking

units are separated in to blocks units are separated in to blocks (groups of units that are known (groups of units that are known before the experiment to be similar before the experiment to be similar in some way that is expected to in some way that is expected to affect the response to the affect the response to the treatments, e.g. blocking by gender treatments, e.g. blocking by gender or race); random assignment of units or race); random assignment of units to treatments is carried out to treatments is carried out separately within each blockseparately within each block

BlockingBlocking

12 plantsFrom store A

4 plants

4 plants

4 plants

No fertilizer

½ dose of fertilizer

Full Fertilizer

Compare quality

4 plants

4 plants

4 plants

No fertilizer

½ dose of fertilizer

Full Fertilizer

Compare quality

12 plantsFrom store B

24 plants

Blocking and controlBlocking and control

Blocking increases our controlBlocking increases our control• Eliminated the effect of origin of the Eliminated the effect of origin of the

plants as a cause for the growth plants as a cause for the growth

Matched PairsMatched Pairs

Creat blocks of units that are as Creat blocks of units that are as closely alike as possible and closely alike as possible and randomly select which one receives randomly select which one receives the treatment OR one subject/unit the treatment OR one subject/unit who receives two treatments in who receives two treatments in random order (a person serves as random order (a person serves as their own control) their own control)

A manufacturer of boots plans to conduct an experiment to compare a new method of waterproofing to the current method. The appearance of the boots is not changed by either method. The company recruits 100 volunteers in Seattle, where it rains frequently , to wear the boots as they normally would for 6 months. At the end of the 6 months, the boots will be returned to the company to be evaluated for water damage.

Diagram a completely randomized experiment for this situation

DiagramDiagram

100 Volunteers

Waterproof Boot

Regular boot

Compare wear

50 people

50 people

What are some sources of What are some sources of variation in this experiment?variation in this experiment?

Some people work inside and others Some people work inside and others outsideoutside

Some people may have more active Some people may have more active lifestyles than otherslifestyles than others

Some people may forget to wear Some people may forget to wear their bootstheir boots

Some people may be more or less Some people may be more or less careful if they think they have the careful if they think they have the new bootnew boot

Matched Pairs DesignMatched Pairs Design

100 Volunteers

Randomly choose one foot to wear Waterproof Boot and the other to wearRegular boot.

Compare wear