Dailyactivities

8:00 A.M. Leave home

8:40 A.M. Arrive at work

2:00 P.M. Have lunch

Go to the dentist

4:45 P.M. Leave dentist

5:30 P.M. Go shopping

Leave shopping

7:20 P.M. Arrive home

HOUSEHOLD#2375

LOCATIONS

Setting the population in motion requires assigning locations to every household's activities. Land-use data for buildings, parking lots, parks and other places were associated with 180,000 locations in the model, providing estimates of the number of people performing various types of activities there. Activities were anchored to individuals' work or school locations, and then places were chosen for additional activities, such as grocery shopping or recreation, taking into account their distance and other measures of their appeal.

Age:

28

27

7

3

Income:

$3?K

$28K

$0

$0

Status:

worker

worker

student

day care

Auto:

n/a

Most metropolitan planning offices conduct detailed traveler activity surveys for small population samples of a few thousand. These logs track the movements of each household member over the course of one or more days, noting the time of each activity. By matching the demographics of survey respondents to the entire synthetic population, realistic daily activities can be generated for every synthetic household member.

LOCATIONS

Setting the population in motion requires assigning locations to every household's activities. Land-use data for buildings, parking lots, parks and other places were associated with 180,000 locations in the model, providing estimates of the number of people performing various types of activities there. Activities were anchored to individuals' work or school locations, and then places were chosen for additional activities, such as grocery shopping or recreation, taking into account their distance and other measures of their appeal.

at work, with each encounter having a different duration, proximity and purpose. During lunch or a shopping trip after work, Ann might have additional short contacts with strangers in public places before returning home.

We can visually represent Ann's contacts as a network with Ann in the center and a line connecting Ann to each of them [see box on next page]. All Ann's contacts engage in various activities and meet other people as well. We can represent these "contacts of contacts" by drawing lines from each—for example, Ann's colleague named Bob—to all his contacts. Unless they are also contacts of Ann, Bob's contacts are two "hops" away from Ann. The number of hops on the shortest path between people is sometimes called the graph distance or degree of separation between those people.

The popular idea that everyone on the earth is connected to everyone else by at most six degrees of separation means that if we continued building our social network until it included everyone on the planet, no two people would be more than six hops from one another. The idea is not strictly true, but it makes for a good story and has even led to the well-known game involving the social network of actors who have appeared in films with Kevin Bacon. In academic circles, another such social network traces mathematicians' co-authorship connections, with one's "Erdos number" defined by graph distance from the late, brilliant and prolific Paul Erdos.

Other types of networks, including the Internet, the links among scientific article citations and even the interactions among proteins within living cells, have been found to display this same tendency toward having "hubs": certain locations, people or even molecules with an unusually high number of connections to the rest of the network. The shortest path between any two nodes in the network is typically through one of these hubs, much as in a commercial airline's route system. Technically, such networks are called "scale-free" when the number of hubs with exactly k connections, N(k), is proportional to a power of k [see "Scale-Free Networks," by Albert-Lâszlo Barabâsi and Eric Bonabeau; Scientific American, May 2003].

Because a scale-free network can be severely damaged if one or more of its hubs are disabled, some researchers have extrapolated this observation to disease transmission. If infected "hub" individuals, such as the most gregarious

Renewable Energy

Renewable Energy

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable.

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