Based on the guides in GN 00302.165, the first census record taken after birth is normally a document with high probative
value. It is a very old record made at a time and for a purpose when there was no
reason to falsify age. In addition, the age was usually given by a responsible member
of the household who would be able to give an accurate age for a small child in the
However, this record, like any other type of record, has a certain incidence of error.
The pattern and extent of this error has been defined for the 1910, 1920, and 1930
The 1910, 1920, and 1930 census records ordinarily show the correct age as of the
enumeration date (see GN 00302.710). When the correct age is not shown, studies indicate that the age shown is almost
always one year from the correct age and that, in these cases, when all evidence in
file is consistent, the alleged DB is correct.
This error pattern seems to be the result of the recordation of an age (rather than
a specific DB) and the recording of that age in some instances as of the date of the
enumerator's visit rather than as of the Census Day.