There are some important things you may need to know about some special issues
involving certain variables.  See
<a href="http://dss.ucar.edu/datasets/ds353.4/docs/more.summary.html#SLP" >this</a>
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NCEP does some Quality Control (QC) on the data, and they set some QC flags in
the reports to qualify, but not quantify their corrections.  After the data is
used to prepare the FNL analyses, no additional corrections are made.
Radiosonde data corrections are described in <a href="http://www.emc.ncep.noaa.gov/mmb/data_processing/data_processing/" >Observational Data Processing at NCEP</a></font>
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Older data is not reprocessed with newer procedures.  DSS does not do QC, but
will investigate user complaints about data values.