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Common questions and answers.
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Extract is a program that allows one to pull off a subset of cases and variables from a large data set. This means that the user does not have to download and maintain the entire file on a hard-drive. It also saves the user the trouble of writing a program to manipulate the hierarchical nature of the data.
Extract works with microdata files - usually from the Census of Population and Housing, but also including Current Population files from the Bureau of Labor Statistics. We will be adding files to this web-interface. So far we are only supporting data from the two most recent censuses (2000 and 1990).
Microdata files are individual records - not summary statistics. Microdata allows a user the flexibility to make custom tabulations, recode variables, and model individual behavior.
If the user is interested in standard statistics such as the poverty rate for a county or the number of females in the state of Michigan, summary files are probably the better choice.
Hierarchical files are the most efficient way to store data. However, they are not easy to use. Thus, the output file from the program will be written in rectangular form.
To analyze the data, the user will most likely use a statistical package such as SAS, Stata, or SPSS. If you are unfamiliar with statistical packages, you probably do not want to use Extract.