What is "Background"?

[elizabeth_algutifan@wssrap-host.wssrap.com]

     Radsafers, 
     I'd like to rehash the gross alpha issue from a few days ago (since 
     I'm currently immersed in it and have been for several weeks now).  
     I'm not sure why it was initiated this way, but when we compare our 
     annual average perimeter and off-site radioactive airborne particulate 
     concentrations to "background" to determine any statistical 
     significance of the station data, we compare the averages to an 
     average 2-year background concentration.  We currently use a 
     one-tailed Student's t-test at the 95% confidence level.  Does the use 
     of a 104-week background sound strange? What time period do other 
     sites use to describe a long-term background average?  Thanks in 
     advance.
     
     Elizabeth Algutifan
     Jacobs Engineering Group 
     at the Weldon Spring Site
     St. Charles, Missouri
     elizabeth_algutifan@wssrap-host.wssrap.com

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