- Observational Study: An observational study observes individuals and measures variables of interest but does not attempt to influence the responses. The purpose of this study is to describe some group or situation.
Observational studies have a population and sample. 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, which is used to draw conclusions from the whole.
Population and Samples Example Problem:
1.7 For each of the following sampling situations, identify the population and the sample as exactly as possible.
(a) A furniture maker buys hardwood in large batches. The supplier is supposed to dry the wood before shipping. The furniture maker chooses five pieces of wood from each batch and tests their moisture content. If any piece exceeds 12% moisture content, the entire batch is sent back.
Answer: So in this situation the population is all the batches of wood being shipped because it is the entire group that we want information from. The sample would be the five pieces of wood that are selected from each batch because this is the portion of the entire population that we are gathering our information from. The sample then tells us information about the whole.
(b) An insurance company wants to monitor the quality of its procedures for handling loss claims from its auto insurance policyholders. Each month the company selects a sample from all auto insurance claims filed that month to examine the accuracy and promptness with which they were handled.
Answer: The population in this case would be all the claims filed with the insurance company within a given month. This is because it is the entire group we want information from. The sample is the selected claims for that month, because again, it is the portion of the whole that we are gathering information from.
-Census: A census is a sample survey that attempts to include the entire population in the sample. So it is like an observational study that includes everyone in the population.
Key Point: A census is not full proof. A census can only attempt to sample the entire population. The Census Bureau for example estimates that the 1990 census missed 1.6% of the American Population.
Link to the 2010 census: http://2010.census.gov/2010census/
The link has all sorts of information about the 2010 census including a more detailed explanation of what it is.
-Experiments: An experiment deliberately imposes some treatment 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. So this is different from a census and an observational study because in an experiment you want a change in behavior.
Experiment Example Problem:
1.13 Before a new variety of frozen muffin is put on the market, it is subjected to extensive taste testing. People are asked to taste the new muffin and a competing brand and to say which they prefer. Is this an observational study or an experiment? Explain your answer.
Answer: This is an experiment because a treatment is imposed on the subjects. The subjects are the people and the treatment would be the new muffin. The person running the experiment is influencing the result of the experiment. The subject is being given a new muffin which will influence the subject's responses.
In conclusions, observational studies, census', and experiments are just different methods of analyzing and collecting data.
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