# The Whole Is Greater than the Sum of the Parts: A Multilayer Approach on Criminal Networks +
## 2 Materials and Methods
### Multilayer Criminal Network Data  
As already explained in Section 1.3, our dataset derives from the Montagna operation, which was an investigation carried out by R.O.S. (Special Operations Group), i.e., a specialized anti-mafia police unit of the Italian Carabinieri. This operation was concluded in 2007 by the Public Prosecutor's Office of Messina, i.e., the third largest city on the island of Sicily (Italy).  
The Montagna operation focused on the [PERSON] family and the [PERSON] clan, i.e., two mafia families who used a cartel of corrupted entrepreneurs to infiltrate the public works in the northeastern part of Sicily in the period 2003-2007. The [PERSON] family also had a mediator role between two families known as [PERSON] and [PERSON], who operated around Messina, and other criminal organizations around Palermo and Catania.  
On 14 March 2007, the Preliminary Investigation Judge of the Court of Messina issued a pre-trial detention order, which represents the main data source for our criminal networks. The pre-trial detention for 38 individuals was ordered by the court who wrote a document of more than two hundred pages containing a lot of details about meetings, phone calls, crimes and other activities among the suspects.  
Two graphs were initially built from the analysis of this document: Meetings and Phone Calls [25; 29; 40]. The Meetings network is made of 101 nodes (i.e., suspected criminals) and 256 edges (i.e., meetings among couples of suspected criminals). The Phone Calls network is characterized by 100 nodes (i.e., suspected criminals) and 124 edges (i.e., phone calls among couples of suspected criminals). A total of 47 suspects jointly belongs to both networks.  
In the current study, we also built a third graph called Crimes characterized by 25 nodes and 74 edges. It shares with the Meetings and Phone Calls graphs 20 nodes, which are also in this case suspected criminals. Individuals are connected by an edge if they have committed crimes together.  
In our previous work [36], we created from the Meetings and Phone Calls graphs a weighted and undirected multilayer network with 154 actors, 439 intralayer edges and 2 layers. In this paper, we rebuilt the multilayer network adding the Crimes layer to the Meetings and Phone Calls layers (see Figure 1). Our new undirected and weighted multiplex network has 226 actors and 454 intralayer edges. Edges in the Meetings layer represent the meetings among suspected criminals; those in the Phone Calls layer refer to the phone communications among distinct phone numbers; those in the Crimes layer refer to common crimes committed by the members of the criminal network. The numberof meetings, phone calls or common crimes are encoded by the edge weights. According to Definition 3, our network can be identified as a multiplex network. In fact, a multiplex network requires that each layer share at least one node with some other layer. In our case, Meetings and Phone Calls layers share 47 actors; Meetings and Crimes layers share 25 actors; and Phone Calls and Crimes layers share 20 actors.