# Characterizing Situations of Dock Overload in Bicycle Sharing Stations
## 3 Methodology
### Knowledge Exploration
The areas with the highest intensity for intermittence situations are mainly located in the World Trade Center (on the bottom of the map), while the highest intensity for critical situations is located both in the areas of the World Trade Center and of the Museum Of Modern Art (on the top of the map). ### Parameter analysis

The main parameters of the OMP-Miner algorithm are as follows: (i) The threshold used to discriminate station occupancy levels into Normal and Overloaded, i.e., the occupancy threshold _full-th_; (ii) the threshold used to decide whether two stations are located nearby or not, i.e., the

Figure 6: Heat maps representing intermittence and criticality values in Barcelona at the hourly time slot (between 11:00 a.m. and 12:00 a.m.). _maxdist_ = 0.5 km, and _time slot size_ = 1 h. _full-th_ = 3. Figure 7: Heat maps representing intermittence and criticality values in New York at the hourly time slot (between 11:00 a.m. and 12:00 a.m.). _maxdist_ = 0.5 km, and _time slot size_ = 1 h. _full-th_ = 3.

maximum distance threshold _maxdist_; and (iii) the time granularity used to analyze the evolution of imbalance situations over time, i.e., _time slot size_. We analyzed the impact of parameters _full-th_, _maxdist_, and _time slot size_ on (i) the cardinality of the mined OMPs (i.e., the number of OMPs per time slot), (ii) the distribution of the intermittence values of the mined OMPs, and (iii) the distribution of the criticality values of the mined OMPs. Moreover, we also analyzed the impact of the day category on the hourly distribution of the intermittence and criticality values for the mined OMPs. In the experimental evaluation, we varied one parameter at a time, and we set the standard configuration for the remaining parameters. The standard configuration was introduced in Section 4.2 as _maxdist_ = 0.5 km, _full-th_ = 3, _time slot size_ = 1 h.

For the sake of brevity, we will hereafter report the results achieved on the Bicing dataset (Barcelona) considered as a reference example study. Similar results have been obtained from the Citi Bike dataset. **Occupancy threshold (_full-th_). ** Figure 8a,b show the impact of the _full-th_ parameter on the mined OMPs. The two figures report the total number of mined OMPs for each range of intermittence and criticality value when increasing _full-th_. A station is in an overloaded condition when less than _full-th_ free docks are available. Therefore, the higher occupancy threshold value we set, the more OMPs with high intermittence/criticality value could be extracted. The results reported in Figure 8a,b show this trend. The number of OMPs for each intermittence and criticality range increases almost linearly when increasing the _full-th_ value. This increase is higher for the intermittence index. **Maximum distance threshold (_maxdist_). ** Figure 9a,b show the impact of the _maxdist_ parameter on the number of mined OMPs. The two figures report the total number of mined OMPs for each range of intermittence and criticality value when increasing _maxdist_. When the _maxdist_ value is increased, the number of nearby stations also increases. Consequently, the number of mined OMPs increases because larger patterns including more stations are also generated. Results show that when increasing _maxdist_ the number of OMPs increases almost exponentially for each intermittence range and almost linearly for each criticality range. However, the number of OMPs that are worth considering for manual inspection (i.e., those with high intermittence/criticality values) remains roughly stable even while enforcing _maxdist_ values higher than 0.5 km. Setting _maxdist_ values higher or equal to 0.6 km is less interesting in our context of analysis because the end users are willing to move to physically closer stations if the expected

Figure 8: Barcelona. Impact of the occupancy threshold on the characteristics of the mined OMPs. _maxdist_ = 0.5 km. _time slot size_ = 1 h.

destination is fully occupied. **Time slot size.