Readme File for Geographic Comparison Tables (GCTs) 

1. gct_readme.txt

This readme file is provided to explain the file naming conventions and general content of the downloaded files.

When applicable, a second data set specific readme file also is provided to explain potential gaps in the data for the selected geographic areas. In many data sets in American FactFinder (AFF), not all data is available for all geographic areas. The specific reason varies with each data set but in general is due to one of the following reasons:

- sampling sizes
- population thresholds
- geographic thresholds for selected tables 	  

Be sure to consult the data set specific readme file when it is provided.

2. Geographic Comparison Table data files and what they contain. 

a) File type extensions: 

The data files are either comma delimited text files or Microsoft Excel (.xls) files. Two types of data files are created:

	gct_<dataset>_data1.<ext> = dynamically produced file with data values
	
	<dataset> = the unique data set name
	<ext> = the chosen file extension (.txt or .xls)


b) Non-iterated Data Files: <dataset>_data1.<ext>

A download will contain one dynamically produced file with data values. This file will be named as gct_<dataset>_data1.<ext>. Each data file includes 2 primary keys to link to other data files:

	GEO_ID: the Geographic Identifier
	SUM_LEVEL: the Summary Level Code

GCTs can only be displayed and downloaded one at a time


c) Iterated Data Files: gct_<dataset>_data001_1.<ext>

These data sets contain tables that are repeated for Race and Ethnic Groups (REGs) and/or Ancestries, for example, Summary Files 2 and 4 from the Census 2000. 

A GCT downloads from this data sets will be named as:

gct_<dataset>_data001_1.<ext>

 
Each file will contain data for one Race or Ethnic Group (REG); the REG is identified by the 3 digit code in the file name *data001*.<ext>. Each file will contain data for only one REG. Iterated data files contain 3 primary keys to link to other data files:

	GEO_ID: the Geographic Identifier
	SUM_LEVEL: the Summary Level Code
	CHARITER: the 3 digit code that corresponds to the Race and/or Ethnic Group(s)  

	
d) Formation of the Geographic Identifier (GEO_ID).
	
The GEO_ID is used in the American FactFinder application to uniquely identify 
each geography.  A GEO_ID is formed by appending a series of text strings:
 3-digit summary level code + '00US' + applicable geocodes.  
For example, the GEO_ID for Autauga County, Alabama would be formed by appending:
 '050' + '00US' + '01' (State FIPS) + '001' (County FIPS) to produce: '05000US01001'.


3) Refer to the Readme_<dataset>.txt for more information

The geographic selections in the *geo.txt file may not have matching data values in the data.txt files. The reasons for this vary by data set. Refer to individual readme files for each data set for further explanation. 

4) Data Set Technical Documentation

The technical documentation is available from the data sets page. Click on the data set radio button and then click on the Technical Documentation link. The data set page can be bookmarked:

http://factfinder.census.gov/servlet/DatasetMainPageServlet?_lang=en