 **Y. Maeno and Y. Ohsawa** 

- b0**'={Abdul A. Al-Omari, Marwan Al-Shehhi, Mohamed Atta, Waleed Alshehri} =b**0. 

- b1**'={Marwan Al-Shehhi, Mohamed Atta, Fayez Ahmed, Waleed Alshehri}.** 
- b2**'={Waleed Alshehri, Abdul A. Al-Omari, Wail Alshehri, Satam Suqami}.** - b3**'={Fayez Ahmed, Mohand Alshehri, Hamza Alghamdi}=b**3. 

## 4.2 Performance Evaluation

In information retrieval, precision and recall are used as evaluation criteria. Precision p is the fraction of relevant data among the all data returned by search. The relevant data here is the records where the covert conspirator has been deleted in the second step. Recall r is the fraction of the all relevant data that is returned by the search among the all relevant data. They are defined by eq(11). and eq.(12). 

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



. (12) 
Besides, F value is useful as a geometric mean of precision and recall. It is defined by eq.(13). 

p r F+ = + =2 ) 1 1 ( 2 1 1 p r pr
. (13) 
F value gain gF **is defined by eq.(14). It is the ratio of the F value of the algorithm to** 
the F value of the random retrieval. 

## Rd

FF
F

## G =

. (14) 
Performance of the algorithm is evaluated with the test data under several conditions. 

Figure 5 shows precision and recall to retrieve the records where a covert conspirator, Mustafa A. Al-Hisawi, has been hidden. Mustafa A. Al-Hisawi was a big financial sponsor to the hijackers, as mentioned in section 1.The number of clusters is |c|=4. The probability of communication transmission is t=0.8. The horizontal axis is the ratio of the number of retrieved basket data to the number of the whole basket data ( m |/ b | ret ). 

The records retrieved as top 10% ranking are correct. The algorithm outputs correct information. The ranking function Isd(bi**) seems to show a little better performance than** Iav(bi). Isd(bi**) is employed in the following study. Precision is 100% when the top 10%** 
of the baskets are retrieved. The algorithm works fine. Precision is 0.45 when the all baskets are retrieved. The problem here includes many correct answers. It is not so difficult because the network is small. (Maeno, 2006) studies the performance for a network consisting of 400 nodes 