## 2 Materials and Methods  
### Research Object  
#### 2.1.1 Time Series  
Time series has the following characteristics: the continuation of the development trend to the future is set as a premise; there is no regularity in the data involved; exclude causal relationships between the development of factors [16]. As an important indicator for studying the development of accidents, the number of accidents determines the future safety status of the entire building construction field to a certain extent. Analyzing and forecasting as a time series can help illustrate accident trends. The number of deaths, like the number of accidents, has a similar development trend, but people tend to pay more attention to the number of accidents in accident prediction, resulting in the impact of the number of deaths in later stages being mostly indirect. Predicting the number of fatalities is more complex than the number of accidents. According to past statistics, in the process of building construction, construction accidents have been high due to the lack of material selection, lack of training and safety awareness of personnel, and chaotic on-site management. Figure 1 shows the number of housing municipal engineering production safety accidents and deaths in China from 2009 to 2019. From 2012 to 2015, the number of accidents and deaths decreased as the state issued and improved the relevant laws and regulations and relevant regulations of the construction industry, standardizing all aspects of the construction industry from design, and construction to acceptance, etc. and improving the management level to a certain extent [17]. However, since 2016, the number of cases and deaths has continued to \"double rise\", which was related to the adjustment of accident reporting methods by the State Administration of State Safety, the lack of work and responsibilities related to the hidden dangers of construction enterprises [18], and soaring housing prices in some parts of the country. Looking at the global accident development trend, it was found that the construction industry or other industries in each country have their own unique industry typical emergencies, and it is precise because of these major shocks that the trend has changed significantly. For example, the impact of the COVID-19 pandemic will exacerbate the pressure on completion in the construction industry, shortening the construction period. At the same time, it will cause more unsafe behaviors and conditions, which increases more accidents. Positive measures need to be taken at an early stage to avoid these phenomena. It is also based on the proven theory that political and economic influences are the primary drivers of right and wrong behaviors. Thus, those special circumstances should be considered when researching, which means that the addition of seemingly unrelated indicators may also have an impact on accident prediction. This is one of the reasons why the nature of the accident is considered when predicting the number of deaths in Section 3.3, and it is an inevitable choice to quantify other influencing factors and highlight their role in forecasting. At the same time, there is a non-linear periodicity in the construction and production safety accidents, resulting in fluctuations in the quarterly and monthly data of accidents. Rather than predicting time series directly, pre-processing can better reflect the real situation. The development of construction accidents is mainly affected by long-term trends, seasonal fluctuations, and other fluctuations which are independent of each other. Choosing an additive model based on locally weighted regression--seasonal and trend decomposition using Loess (STL), dividing the time series into three parts: trend components, seasonal  
Figure 1: 2009–2019 China’s housing municipal engineering production safety accidents. components, and remainders. This method of \"predicting separately and then integrating\" will effectively improve prediction accuracy.