Introduction to Experimental Design Section 1.2

Objectives Discuss what it means to take a census. Describe simulations, observational studies, and experiments. Identify control groups, placebo effects, completely randomized experiments, and randomized block experiments. Discuss potential pitfalls that might make your data unreliable.

In a census, measurements or observations from the entire population are used. In a sample, measurements or observations from the part population are used.

We have to be able to distinguish between observational studies and experiments. In an observational study, observations and measurements do not change the response of the variable being measured. Don’t do anything to the individuals!!! In an experiment, a treatment is deliberately imposed on individuals.

Example–Experiment or Observation… At one time, the silver sword grew abundantly on the island of Maui, but each year there seemed to be fewer and fewer plants. Biologists suspected that goats were partially responsible for the decline in the number of plants and conducted a statistical study that verified their theory.

Example–Experiment or Observation… To test the theory, park biologists set up stations in remote areas of Maui. At each station two plots of land similar in soil conditions, climate, and plant count were selected. One plot was fenced to keep out the goats, while the other was not. At regular intervals a plant count was made in each plot. *This study involved an experiment because a treatment (the fence) was imposed on one plot.

Experiments and Observation… The placebo effect occurs when a subject receives no treatment but believes he/she is in fact receiving treatment and responds favorably. Account for the placebo effect? Two groups… Group 1; receives the prescribed treatment. Group 2; control group, receives a dummy or placebo treatment.

A completely randomized experiment is one in which a random process is used to assign each individual to one of the treatments.

Example-Completely Randomized Experiment Can chest pain be relieved by drilling holes in the heart? For more than a decade, surgeons have been using a laser procedure to drill holes in the heart. Many patients report a lasting and dramatic decrease in angina (chest pain) symptoms. Is the relief due to the procedure, or is it a placebo effect? A recent research project at Lenox Hill Hospital in New York City provided some information about this issue by using a completely randomized experiment.

Example –Completely Randomized Experiment The experimental design can be pictured as The laser patients did well. But the placebo group showed more improvement in pain relief.

Surveys… A common means to gather data about people is to ask them questions which is called a survey. The best surveys utilize yes/no questions or the Likert scale. Open-ended questions are not as good. It can be hard to put a number on those types of questions.

Problems with Surveys…

Choosing Data Collection Techniques… Which technique is best? Depends on the number of variables of interest The level of confidence needed regarding statements of relationships among the variables.

Choosing Data Collection Techniques… Surveys may be the best choice for gathering information across a wide range of many variables.

Choosing Data Collection Techniques… Observational studies are the next most convenient technique for gathering information on many variables. Experiments are the most stringent and restrictive data-gathering technique.

1.2 Introduction to Experimental Design Summarize Notes Read section 1.2 Homework