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Block relationship files contain a part flag field associated with some 1990 and 2000 block numbers. The part flag is stored in the 1990 tabulation block part flag, and the 2000 tabulation block part flag fields of the record layouts. A value of "p" in any of these fields identifies block records that do not have a whole block to whole block relationship. The part flag indicator helps to explain the different types of relationships that exist between the block identifiers. The Census Bureau has created census block relationship information on the basis of one-to-one, one-to-many, many-to-one, and many-to-many block(s).
In some cases, a 1990 census block has a one-to-one relationship with a Census 2000 block. Even though new block numbers have been assigned for Census 2000, the features used to define Census 2000 blocks remain unchanged from 1990. This relationship is considered to be a one-to-one relationship. For example, 1990 census block 101 (in census tract 16) has a one-to-one relationship with Census 2000 block 1000 (in census tract 16.01).
In a one-to-many relationship, a single 1990 census block is now represented by more than one Census 2000 block. For example, 1990 block 102 (in census tract 16) is now represented by Census 2000 blocks 1001 and 1002 (in census tract 16.01). The part flag “p” indicates that only part of 1990 census block 102 is in each of the Census 2000 blocks 1001 and 1002.
In a many-to-one relationship, a number of 1990 census blocks may now be represented by only one Census 2000 block. For example, 1990 census blocks 103, 104, and 105 (in census tract 16) constitute Census 2000 block 1003 (in census tract 16.01).
A many-to-many relationship occurs when more than one 1990 census block is now represented by more than one Census 2000 block. In this case, all 1990 census blocks and Census 2000 blocks in question will have the part flag “p” associated with the block number. For example, 1990 census blocks 201, 202, and 203 (in census tract 16), are represented by Census 2000 blocks 5000, and 5001 (in census tract 16.01).