COMPUTING SCIENCE

# Rumours and Errours

The Rumor Mill

I was curious to see the rumor models in action, and so I wrote a little program. I set up a population of 1,000 individuals, each of whom could be an ignorant, a spreader or a stifler. Initially there was just one spreader and all the rest were ignorants. At the heart of the program was the following procedure, meant to implement the Daley-Kendall model (the one in which pairs of spreaders annihilate each other):

repeatchoose

Xat random from among the spreaders in the population;choose

Yat random from the entire population;

ifYis an ignorant

thenmakeYa spreader

else ifYis a spreader

thenmake bothXandYstiflers

else ifYis a stifler

thenmakeXa stifler

untilthere are no more spreaders

When all the spreaders are gone, nothing more can change, so the
program ends and reports the fraction of the population still
oblivious of the rumor. This fraction, designated *θ*,
should be 0.203188. But the result from my program, averaged over a
few thousand runs, was 0.28 or 0.29—a considerable discrepancy.

At this point, let me pause to say that my big boo-boo had already been committed. Before reading on, you might want to try debugging my algorithm, or even write a program of your own.

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**Of Possible Interest**

**Engineering**: The Story of Two Houses

**Letters to the Editors**: The Truth about Models

**Spotlight**: Briefings