2 Types of Studies Observational Study Experimental Study

Observational Study An observational study observes individuals and measures variables of interest but does not attempt to influence the responses. The purpose of an observational study is to describe some group or situation.

Populations & Samples A population in a statistical study is the entire group of individuals about which we want information A sample is a part of the population from which we actually collect information. Sample data is used to draw conclusions about the population as a whole.

TV ratings Market research is designed to discover what consumes want and what products they use. One example of product research is the television rating service of Nielsen Media Research. The Nielsen ratings influence how much advertisers will pay to sponsor a program and whether or not the program stays on the air. For the national Nielsen ratings: What is the population? What is the sample? What variables are measured?

Census A survey that attempts to include the entire population in the sample. Censuses are often too costly and timely to conduct so we take samples instead. The Current Population Survey (CPS) is one such sample conducted by the Bureau of Labor and Statistics. In it, about 50,000 households are interviewed each month.

Experiments An experiment deliberately imposes treatments on individuals in order to observe their responses. The purpose of an experiment is to study whether the treatment causes a change in the response.

Observational Study or Experiment? A business is studying the relationship between physical fitness and productivity. They divide the employee volunteers into two groups (low and high fitness) and then compare their productivity once the fitness program has been completed. You want to determine what the most popular cars on campus are. To do this, you go to the parking lot and record the types of cars for several samples from different areas of the lot. A insurance company wants to research if there is a difference in the number of children elementary and secondary teachers have. They submit a questionnaire to a random sample of teachers asking them to categorize themselves as elementary or secondary and how many children they have.

Observational Study or Experiment? Is there a cause and effect relationship here? Year Number of Methodist Ministers in New England Cuban Rum Imported to Boston (in barrels) , , , , , , , , , , , , , , , , ,705

Confounding & Causation Just because two variables have a relationship, that doesn’t mean one causes the other. There could be a confounding variable at play. A confounding variable is an additional variable that effects the response but can’t be separated out. Confounding variables are most often found in observational studies. Sometimes the media ignores confounding variables and misinterprets results from observational studies reporting “proven” links when in statistics we only have shown evidence of a relationship.

The Beauty of Experiments Everyone randomly chooses a red or blue chip out of a bag. The red group is assigned to do jumping jacks for two minutes and the blue group is assigned to meditate for two minutes. After the two minutes, their heart rate is measured. Does the treatment cause the difference in heart rate? Since well designed experiments have an element of control, they can be used to imply causation (cause/effect) between the variables.