less than 10%. As with binomially distributed marks, the exam needs to be informative (that is, for marks to be above 50), to be able to extract information from the grade matrix.

Group size
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So far, we have supposed that there are 10 agents who grade each other. We next consider the impact that the size of this group has on the accuracy of the peer grades. We therefore ran an experiment in which we varied the number of agents peer marking. We again use a binomial distribution of marks with a mean of 70. With 5 or more agents, the error of the generalised PeerRank method was less than 5% and was half or less of that of simply averaging the peer grades. With 10 to 20 agents, the error of the generalised PeerRank method was less than a third of that of simply averaging the peer grades. These results suggest that the PeerRank method does not need many peer grades in order to obtain an accurate result. Ideally, we need around 10 grades for each agent, but even with just 5 grades, we are often able to obtain acceptable results.

Biased marks
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Peer grades may be 