## 3 Methodology
### Data Preprocessing
#### 3.2.1 Data Cleaning and Preparation  
We created a clean NBI dataset containing information about bridges from LTBP InfoBridge. During the cleaning processing, we observed missing data and inconsistencies in complying with the FHWA coding guide [17]. Specifically, the early years of the Nebraska data have a large percentage of non-compliant data to the FHWA coding guide. For instance, we observed unrecorded previous reconstruction dates and missing data related to condition ratings. These inconsistencies in the dataset affect the analysis of the data, as it is difficult to account for various maintenance activities. Therefore, we addressed several data-related inconsistencies as follows: The data cleaning consists of handling missing data, removing duplicate records, detecting outliers that can skew the model's understanding of the data, and treating outliers. After the selection of specific attributes, a total of 15,349 bridges belonged to the state of Nebraska from 1992 to 2021. The following are the detailed steps taken to clean the dataset:  
* We removed all missing inspection records with missing deck condition ratings. * For each bridge time series, we detected the changes in the year of the bridge. To preserve the consistency in the bridge life-cycle, with the change in the year a bridge was built, we reintroduced the remaining bridge time series with a new structure number that contains the original structure with a segment suffix. * We discarded inspection records for culverts. * We discarded inspection records of bridges with structure lengths under 20 feet. * We selected a bridge deck with a wearing surface (NBI item 108A): Monolithic Concrete, Gravel, Bituminous, Wood or Timber, Other, and Low-slump concrete. After the selection of specific attributes, a total of 9,152 bridges belonged to the state of Nebraska from 1992 to 2021. This study examines 14 variables from five categories: Physical, Region, Structural Type, Environmental, and Service, as shown in Table 1. These variables can be classified into two types: numerical (either continuous or discrete) and categorical (either nominal or ordinal). We present descriptive statistics for numerical variables to summarize their characteristics in Table 2.  
On the other hand, categorical variables such as material, membrane type, deck protection, owner, and maintenance are divided into different categories or levels, as shown in Table 3. By presenting these descriptive statistics and summarizing the categories within each variable, we provide a comprehensive overview of the data, enabling readers to understand the characteristics and distributions of the variables under investigation. Table 3 reveals an observable imbalance in the distribution of certain categorical variables within the context of Nebraska. Notably, there is a significant preponderance of bridges with \"None\" for deck protection and membrane type (consisting of 81% of bridge decks). Additionally, the most frequently employed bridge material in Nebraska is steel. Furthermore, the ownership of a majority of the bridges falls under the purview of County Highway authorities, accounting for 77% of bridges.