## 4 Results and Discussion
### Partial Dependence Plot  
PDPs were employed to elucidate the effect of individual predictors on the dependent variable by using the Extra Trees Regressor (Figure 4). The x-axis displays the value of independent variables, and the y-axis represents the normalized value of the number of crime reports per area. The PDPs in this study were generated based on the LSV results,which had a mixture of colors near 0, meaning that a non-linear relationships may exist between the selected predictors and the outcome. Within the socioeconomic features, the living population and youth sales ratio showed relatively high importance. The living population mostly showed a linear relationship with the number of crime reports per area over the years. In 2019, there was a downward trend in crime calls until the living population surpassed two million. However, it continued to expand after that, with a significant increase when it exceeded four million. After the COVID-19 breakout in 2020, the growth slope became relatively mild, and crime calls almost fell to half of that in 2019 in 2021. This finding leads us to the conclusion that safety measures should be presented in each commercial district in accordance with the living population, as opposed to uniform options from the central government. On the other hand, the youth sales ratio had a clear non-linear relationship with crime calls. For all three years, crime reports reduced to 30% and sharply increased, reaching up to about 42%. After  
Figure 4: PDP results of standardized commercial crime report density. that, crime reports rapidly decreased, and the dropping rate was relatively small in 2021. This tells us that crimes are positively correlated, particularly when the youth sales ratio is between 30 and 42%. Thus, preventative policies are suggested for certain commercial districts for those that frequently exhibit similar youth sales patterns. Although the PDP was not shown for the land appraisal value due to its obvious linear association with crime, crime reports decreased as the land appraisal value grew over all years. This is in line with earlier research, which showed that crime rises with wealth disparity, suggesting that more affluent communities are less likely to experience property crimes such as burglary and carjacking [50]. Among neighborhood characteristics, all three years of the commercial-to-housing ratio showed a similar trend. Crime calls increased as the ratio exceeded 150%. Particularly, relatively fewer crime calls were reported when the commercial-to-housing ratio was around 25% or less in 2020, and this indicates that crimes decreased in commercial districts with a high residential proportion after COVID-19. The social atmosphere of refraining from going outside due to the fear of infection after the outbreak made commercial districts with a higher proportion of housing have fewer visitors and may have reduced the crime occurrence. In 2021, there was an increase in trips to commercial districts near residential neighborhoods for purchasing fundamental necessities, which led to an increase in crime reports where the commercial-to-housing ratio was low. This finding corresponds to [18], suggesting that an area is safer when the proportion of dwellings is higher and recommending that we should prepare prevention policies by separating into \"residential-oriented commercial districts\" and \"market-oriented commercial districts\". Furthermore, commercial districts that have more than a 150% commercial-to-housing ratio should be more cautiously managed by police to flexibly respond to the next pandemic. The land use mix index appeared to show a relatively linear relationship with crime: the frequency of crime reports per area rose as the land use mix index increased, especially when the value was greater than 0.3. This result implies that a lower land use mix has a positive effect on crime reduction. Gathering facilities revealed a non-linear relationship, with a rapid rise in crime calls up to around 35 facilities and subsequent maintenance after that level.