Delivery ratio of the messages,Average message delivery latencyOverhead ratio (of the underlying routing mechanism) Suppose that $M$ be the set of all messages created in the network and $M_d$ be the set of all messages delivered. Then, the delivery ratio is computed as $|M_d| / |M|$.
Now let the $i^{th}$ delivered message was created at time $c_i$ and delivered at time $d_i$. Then the average message delivery latency is computed as $(\sum_{i = 1}^{|M_d|} (d_i - c_i)) / |M_d|$. Note that, in Statistics, mean, median and mode are all the measures of average. But "loosely speaking", unless otherwise specified, we refer to the "mean" value when we say "average." Nevertheless, the MessageStatsReport in the ONE simulator provides a measure of both the mean and median values wherever appropriate.