Segregation Simulator Help
This segregation applet simulates the movement of people between houses within a "city." The city is divided into 10 x 10 "tracts" which are in turn divided into a grid of smaller squares. Each small square in the city represents a house that can be vacant (white) or occupied by a red or blue or, in three-color mode, a green square. If a square has a yellow background it is restricted, in some way, to only certain colors.
Running Controls - Start, Stop, Reset, Shuffle, Counters
The Start and Stop buttons begin and pause the simulation while the drop-down menu next to the Stop button changes the speed of the simulation. The Reset City button restores the arrangement of houses to the state at the start of the previous simulation. The shuffle houses button distributes the colors randomly throughout the city.
The Reset Counters button resets the Move/Attempts counters to 0. The Move/Attempt counters display the numbers of moves that have actually occurred in the simulation and the number of attempted moves. An attempted move will not result in an actual move if a house is selected to move into a new location forbidden by the current moving rules (see below).
Indices of Segregation
The Indices of Segregation provide a numerical representation of how segregated the city is.
Index of Dissimilarity (D)
The index of dissimilarity is the most common measure of segregation. The value of D represents the proportion of a group which would need to move in order to create a uniform distribution of population. There is a value of D for each pair of colors. For example D(R,B) represents the dissimilarity for red and blue. A value of 0.25 would indicate that 25% of the red (or blue) population would need to move to completely integrate red and blue houses in the city. A D value of 0 represents a completely integrated city while a value of 1 indicates complete segregation.
Interaction or Exposure Index (B)
Strictly speaking, theses indices are NOT measures of segregation, but of isolation. They provide some measure of the probability that a member of one group will meet or interact with a member of another group. A value of B(R,G) = 0.36 suggests that 36% of the interactions of individuals from the red group will be with members of the green group. These indices are not symmetric so that B(R,G) does not equal B(G,R). The maximum value of B depends both on the distribution of ethnic groups AND on the proportion of minorities in the city. Generally speaking, the value of this index will be highest when the two groups have equal numbers and are spread evenly among tracts.
The Entropy Index (H)
The entropy index is based on how spread out the ethnic populations are among the city tracts. The maximum value of H is 1, when each tract contains only one group. That is, cities with higher values of H have less uniform ethnic distributions. Typical values for American cities were around 0.60 in 1980. The minimum value of H is 0, when every tract has the same composition as the whole city. That is, cities with lower values of H have more uniform ethnic distributions.
For further discussion see the Racial Residential Segregation Measurement Project. The indices used for the simulator are based on Michael J. White, 1986, "Segregation and Diversity Measures in Population Distribution." Population Index, Vol. 52, 198-221. Click here for a more detailed description (pdf file).
The 3x3 grids in the Moving Rules section visually depict the preferences that control moving to another square. The middle square in each grid indicates the color of the house to be moved, and the number of colored squares around the middle signifies the number of neighbors of a particular color preferred and tolerated (minimum of zero, maximum of 8. The location of these squares around the middle is irrelevant; only the total number is important.) You can change the squares from vacant to occupied (and vice versa) by clicking on the squares surrounding the center house in each grid.
This controls the minimum number of like-colored squares surrounding the moving house and represents preference for similar neighbors. For example, having zero squares filled in around the red house means that a red house will move to a location with zero to 8 red neighbors. Having 2 squares filled in means that a red house will only move to a location that has 2 to 8 red neighbors.
This controls the maximum number of differently-colored squares surrounding the moving house and represents prejudice against dissimilar neighbors. For example, having zero blue squares filled in around the red house means that a red house will not move to a square with ANY blue neighbors (i.e., absolute intolerance). Having 4 blue squares filled in means that a red house will move to a location that has 0 to 4 blue neighbors. Having all 8 squares filled in means that a red house will move to a location that has 0 to 8 neighbors (i.e., complete indifference.)
This regulates the distance that a house will move within the city grid. Unlimited means that a house can potentially move any distance within the city. A limit of 10 means that a house will not try to move more than 10 units away from its original location, etc.
The size of the city grid can be selected with the City Grid Size control (10x10 to 120x120).
The city can be populated with two (red & blue) or three (red, blue, & green) different house colors.
The percentages of house colors can be adjusted using the slider controls. Note that the sum of the percentage of each color and the vacancy rate always equals 100%. You may lock a percentage to stay fixed while adjusting other sliders by selecting the "Lock" box next to a slider. Once you have adjusted the sliders, you must press the "Apply and Shuffle" button to change the city population to the slider settings. This button will also randomize the location of the houses in the city.
The drawing tools allow you to change colors on the grid or to create vacancies. The "Pen" function changes one house at a time; the "Rectangle" function changes blocks of houses simultaneously.
You can create zones (using either the Pen or Rectangle function) that prevent particular colors from moving into the area. (Houses of the excluded color that are already in the area will stay there until the simulator moves them to a different, suitable location.) Exclusive areas are yellow.
Erase Exclusive Area:
Use the erase exclusive area control to remove exclusive areas. This will allow previously excluded colors to move into the selected area.
You can create a barrier, or uninhabitable zone, by first vacating the desired area and then excluding all colors from the area.