Types of Experimental Designs EQ: How can I make a BETTER experiment?

Observational Study: a study based on data in which no manipulation of factors 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

Terminology 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 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 Design Control 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 Replicateover as many subjects as possible.

Analyzing Experiments Aspirin 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 pill The men will be randomly assigned to either the treatment group or placebo group. Each treatment will be replicated 500 times Factor:AspirinResponse:Number of heart attacks Levels:Low dose and none (Placebo)

Randomized Experiment Design Aspirin Study Low dose aspirin 500 menPlacebo Compare number of heart attacks in each group 500 men 1000 volunteers GroupsTreatments Random Assignment

Cautions Lack of Blindness If the experimenter assessing the responses to treatments knows which treatment was given, he/she may bias the resultsIf the experimenter assessing the responses to treatments knows which treatment was given, he/she may bias the results if a subject knows that they got the placebo treatment, they may under-report their response if a subject knows that they got the placebo treatment, they may under-report their response Lack of realism subjects or treatments or setting of an experiment may not realistically duplicate the conditions we want to study

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

Optigro 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 No fertilizer ½ dose of fertilizer Full Fertilizer Compare quality

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

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

Blocking 12 plants From store A 4 plants No fertilizer ½ dose of fertilizer Full Fertilizer Compare quality 4 plants No fertilizer ½ dose of fertilizer Full Fertilizer Compare quality 12 plants From store B 24 plants

Blocking and control Blocking increases our control Blocking increases our control Eliminated the effect of origin of the plants as a cause for the growthEliminated the effect of origin of the plants as a cause for the growth

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

Diagram 100 Volunteers Waterproof Boot Regular boot Compare wear 50 people

What are some sources of variation in this experiment? Some people work inside and others outside Some people work inside and others outside Some people may have more active lifestyles than others Some people may have more active lifestyles than others Some people may forget to wear their boots Some people may forget to wear their boots Some people may be more or less careful if they think they have the new boot Some people may be more or less careful if they think they have the new boot

Matched Pairs Design 100 Volunteers Randomly choose one foot to wear Waterproof Boot and the other to wear Regular boot. Compare wear