Each of the three Summary File directories include zipped files containing estimate files (file names beginning with an "e") and margins of error files (file names beginning with an "m"). The estimate files contain published ACS estimates and the margin of error files contain published ACS margins of error for their respective estimates. Here is the naming convention used for those files:
The estimates and margins of error for Detailed Tables are grouped together in by sequence numbers, as discussed in Chapter 2.3. There is an estimate and margin of error file for each sequence number.
The format of the estimate and margin of error files are identical; they are strings of comma- delimited ASCII text. Each row represents a different geographic area and the first six fields contain metadata such as the geographic area and the sequence number. Following those fieldsare the estimates or margins of error for the Detailed Tables. Starting and ending positions of the fields associated with each Detailed Table can be found using the Sequence Number and Table Number Lookup file, which is discussed in Chapter 2.3. The estimates or margins of error for one Detailed Table span several fields within a row.
Here is the record layout of the estimates and the margin of error files:
Field Name |
Description |
Field Size |
FILEID |
File Identification |
6 Characters |
FILETYPE |
File Type |
6 Characters |
STUSAB |
State/U. S. - Abbreviation (USPS) |
2 Characters |
CHARITER |
Character Iteration |
3 Characters |
SEQUENCE |
Sequence Number |
4 Characters |
LOGRECNO |
Logical Record Number |
7 Characters |
Field # 7 and up |
Estimates |
Various |
Going back to the example from Chapter 2.3, we know that table B08406 corresponds to sequence "0003". Additionally, the Sequence Number and Table Number Lookup file (as shown earlier) tells us that table B08406 begins at position seven and contains 51 cells.
In order to get estimates for Maryland; Allegany County, MD; and Anne Arundel County, MD one must recall the logical record numbers associated with each geography. In Chapter 2.4, we identified these to be "0000001", "0000012", and "0000013", respectively. The logical record number, LOGRECNO, must be used to merge the geography information to the estimate and margin of error files.
The example below shows the estimate file for sequence "0003" and all geographies except census tracts and block groups for the state of Maryland using the 2010 ACS 1-year Summary File. Note that each row has a uniquely assigned logical record number, called LOGRECNO, which links the estimate to a specific geographic area. The pictured example has the logical record numbers corresponding to Maryland, Allegany County, and Anne Arundel County circled. Estimates for table B08406 at these geographic levels can be found within their respective rows at field seven and continuing for 50 additional fields.