# Characterizing Situations of Dock Overload in Bicycle Sharing Stations
## 3 Methodology
### Knowledge Exploration
The intermittence measure of an OMP is computed to measure the presence of a dock overload condition from the occupancy levels of the corresponding stations (see Algorithm 1, line 7). The higher the intermittence value, the more severe the imbalance condition. Hence, OMPs with highest intermittence values should be considered first in the result exploration. Tables 3 and 4 report some examples of top ranked OMPs with maximal intermittence value extracted from the Bicing and Citi Bike datasets, respectively. In both tables, OMPs are sorted by decreasing intermittence value. The example OMPs from the Bicing dataset (Table 3) are characterized as follows. OMPs with identifiers (IDs) 5-7 represent dock overload conditions that could yield a limited disservice for end users. Each of these OMPs represents a group of stations that the end user is likely to find fully occupied in alternate fashion (in about 62-63% of the recorded timestamps according to the intermittence value). However, the low criticality values of these OMPs point out that the stations in each OMP are rarely fully occupied at the same time (in about 0.13-1.56% of the cases). It follows that, in case the user is unable to park in one station, she/he can move to another nearby station where free parking docks will be available with a high probability. For example, OMP with ID 5 indicates that the usage levels of stations _Carrer de Bonavista_ and _Pl. del Poble Romanti_ are critical in an alternate fashion from 7:00 a.m. to 8:00 a.m. in 63% of the cases, but they are fully occupied at the same time only in 1.56% of the cases. On the other hand, OMPs with IDs 1-2 represent dock overload conditions that could result in _a more serious disservice_ for end users. Each of these OMPs models a group of stations having both intermittence and criticality values higher than OMPs with IDs 5-7. For each OMP, at least one station has a high probability of being occupied (intermittence value higher than 71%), and all stations have a non-negligible probability of being fully occupied at the same time (criticality about 8%). Therefore, in case the user cannot park in one station, she/he might not find a free dock at a nearby station approximately 8% of the time. As an example, OMP with ID 1 shows that, from 4:00 a.m. to 5:00 a.m., stations _Vilamara davant_, _Mallorca_ and _Calabria_ have a critical usage level in an alternate fashion in 73.84% of the recorded timestamps, and they are simultaneously fully occupied in 8.29% of the cases. OMPs with IDs 3-4 represent an intermediate condition between the two above. These OMPs have intermittence and criticality values higher than OMPs with IDs 5-7 (intermittence 70-71% instead of 63% and criticality 1.86-4% instead of 0.13-1.56%), but lower than OMPs with IDs 1-2 (intermittence 70-71% instead of 73% and criticality 1.86-4% instead of 8%). Based on the mined knowledge, domain experts may recommend an alternative nearby station for parking and/or targeted maintenance actions. For instance, they may decide to relocate bicycles at the beginning of the time slot, moving them from stations with critical levels to non-critical stations. Compared to the OMPs extracted from the Bicing dataset, the top ranked OMPs mined from the Citi Bike dataset have very high intermittence values (between 90% and 100%) and criticality equal to 0% (Table 4). For example, OMP with ID 2 consists of four nearby stations (_[W 33 St & 8 Ave, W 29 St & 9 Ave, W 31 St & 8 Ave, Penn Station Valet]_) with 100% intermittence and 0% criticality from 8:00 p.m. to 9:00 p.m. These stations are close to Madison Square Garden Stadium and Pennsylvania Station, which are big subway and train hubs. These OMPs indicate conditions which could lead to a limited disservice for the end users. On the one hand, since the OMP intermittence value is very high, at least one of the stations in the OMP is likely to be fully occupied, while, on the other hand, since the criticality value is 0%, at least one station has a free dock in all the recorded timestamps.