**The Geographic Areas We
Used**

** **

**
**To
measure racial residential segregation, you need to make four
decisions:

· *Which one race or two
are of interest to me?*

· *What is the largest
geographic area of interest to me?
Is it a city, a county or a metropolis?*

· *What component parts of
the larger geographic area are to be used in calculating measures of
segregation?*

*
*· *Which measure of
segregation best satisfies my needs: the ***index of dissimilarity, **an
**isolation **measure or an **exposure or interaction
**measure?

** **

**Geographic Units Used in the
Census of 2000**

* *

# Geographic
Category |
# Number of Units in
Category |
# Average
Population |

# |
# |
# |

# Entire
Nation |
#
1 |
#
281,421,906 |

# Regions |
#
4 |
#
70,350,000 |

# Divisions |
#
9 |
#
31,266,667 |

# State |
#
50 |
#
5,628,000 |

# Metropolitan
Areas |
#
331 |
#
682,725 |

# Congressional
Districts |
#
435 |
#
646,897 |

# Counties |
#
3,141 |
#
89,589 |

# Census
Tracts |
#
66,304 |
#
89,589 |

# Block
Groups |
#
211,267 |
#
1,332 |

# Blocks |
#
8,262,363 |
#
34 |

# |
# |
# |

#

#

*Large Geographic Areas for Which Segregation Measures are Available
*

** **

This website provides you
will the ability to obtain racial residential segregation scores for: *Entire states*

*
Each of the 331 metropolitan areas*

*
All central cities and other cities with 100,000 or more residents in
2000*

* *

*Component Geographic Units
Used in the Calculation of Segregation Measures*

* *

**
**For
purposes of making sure that all housing units are enumerated in the decennial
count, the Census Bureau designates census tracts. These are contiguous geographic areas
containing an average of 4200 residents.
They may be thought of as neighborhoods but they were delineated for
purposes of census taking so they may or may not correspond to what residents
think of as neighborhoods.

For enumeration purposes, the Census Bureau also delineates Block Groups
made up of contiguous clusters of blocks.
These contained an average of about 1,300 residents in 2000. Finally, the Census Bureau presented
findings for blocks. In
metropolitan areas, these are typically city blocks bounded by four streets but
they might be gore shaped areas bounded by three streets or might have a river
or railroad as a boundary. On
average, blocks in 2000 contained just 34 residents.

*How to Select a Component
Geographic Area?*

** **

**
**The
numerical value of segregation measures depends upon the geographic units used
in their calculations. A census
tract might contain some blocks that are exclusively white while other blocks
are exclusively black. If this is a
common occurrence in a metropolis, than a measure of segregation calculated from
census tract data will be numerically larger than a measure of segregation
calculated from block data.

This website gives you your own choice of geographic units. Traditionally, most segregation scores
have been calculated using census tract data, partly because of the availability
of such data and because it was simpler and faster to use fewer geographic units
in the pre-computer days. But
census tracts are generally larger than the neighborhoods that most people have
in mind when they think about residential segregation since most census tracts
are home to more than 4,000 people.

This implies that block groups are highly appropriate for the calculation
of segregation measures.

This website provides you with information about the number of census
tracts, block groups and blocks that make up the larger geographic area for
which segregation measures are calculated.

*An Important Note of
Caution*

** **

**
**The
*index of dissimilarity *is statistically independent of the
relative size of the two groups used in its calculation. However, its value may be distorted if
the number of geographic units used in its calculation exceeds the number of
people in one of the two racial groups whose distributions are compared. If the geographic distributions of
blacks are whites are compared but the analysis is done with block data for a
metropolis in which there are 15,000 blocks but only 3,000 blacks; there will
necessarily be some block with no African American residents. For that reason, *indexes of
dissimilarity *must be interpreted with great caution if the size of one
of the racial groups using in the calculation is less than five times the number
of areal units used in the same calculation. This implies that *indexes of
dissimilarity* calculated from census tracts or block groups are often
more valuable and accurate descriptions of racial residential segregation than
*indexes of dissimilarity* calculated from block
data.

The *isolation index *and the *exposure or interaction
*measures of segregation are not confounded in situations where one
racial group is smaller than the number of areal units used in the
calculation.

Some counties in 2000 had very small populations so they included only
one census tract or one block group.
In such counties, indexes of racial residential segregation could not be
calculated since every person in the county lived in the same geographic
unit.

** **

*
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