Update background.markdown

Author: altergc

SDTH/doc/content/background.markdown

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 Programs and scripts used to manage data use for statistical analysis are often long and complex.  It can be very difficult to determine how a variable was created or modified.  This problem is compounded by the differences among languages used to manage data.  Each statistical software package has its own syntax, and experience with one language may not help understanding a different one.
 
 SDTH was created to provide a simple way to describe the key features of a data transformation script.  SDTH answers questions such as: “Which variables affected variable X?” and “Which variables were affected by file Y?” SDTH focuses on how data entities (variables, dataframes, and files) are related to each other and to the steps in a program that change them.  
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