ask-a-friend marketplaces

Speaking of ktm readers solving people's problems, this SCIENCE NEWS article, on Ask-a-Friend Marketplaces, is interesting (subscription may be required)

In a 2003 experiment, sociologist Duncan Watts of Columbia University and his coworkers used e-mail to test the notion that every person in the world can reach any other person through a chain of just a handful of social ties. In many instances, the chain fizzled out when someone failed to pass the message on.

The high attrition rate demonstrated in the experiment suggests that it may be difficult to find "faraway" information in a social network. Many people may not be motivated enough to participate.

To overcome this problem, the idea is to offer a reward for the information. Then, as a query is passed from person to person, each participant takes a cut. The search continues until the reward money runs out or an answer is reached.

[snip]

The crucial parameter describing the underlying network is its "effective branching factor." In effect, it's the average number of friends to whom a member of the network passes on a query.

When the branching factor, b, is greater than 1, there exist reasonably short paths to the answer. However, if the branching factor is less than 2, the amount you have to pay in incentives to get an answer is prohibitive—even though short paths to the answer exist. The situation is much more favorable when the branching factor is greater than 2.

"For a large branching factor, the propagation of queries is very efficient in its use of reward," Kleinberg and Raghavan conclude in a paper presented at the 46th Annual Symposium on Foundations of Computer Science, held recently in Pittsburgh.

"By simulation, we have found that the transition at b = 2 is already apparent for rarities n of moderate size," the researchers report. A rarity of n means that one out of n members possesses the answer to a given question.

So, when you're offering incentives, it's a good strategy to ask at least two friends when you need advice. Asking just one friend isn't enough to give you a reasonable chance of eventually getting an answer via some chain of acquaintances.