

![0_image_1.png](0_image_1.png)

![0_image_3.png](0_image_3.png)

![0_image_0.png](0_image_0.png)

![0_image_2.png](0_image_2.png)

constant. The method can be applied for large networks. Note that several hours of calculation is necessary with a standard personal computer when N approaches to 1000. The size of the network is limited by the amount of calculation rather than by the accuracy obtainable from the method.

Fig.7 shows the F measure F ( D t ) as a function of the number of the observed data D in the trial where the node n 0 in the model (B) is the target covert node to discover (C = { no}).

The horizontal axis is the ratio of D to the number of the possible communication patterns ( | OUC | = N+1) as assumed in IV-B. The ratio was D/N = 1 in Fig.4 through Fig.6.

The broken lines indicate the theoretical limit and the random retrieval limit. The F measure is close to the theoretical limit, if more than 80% of the possible communication patterns is observed. The performance is no better than that of the random retrieval if only 50% of the possible communication patterns is observed. It is a major restriction imposed on the method that many of the possible communication patterns need to be