## 3 Method
### Ism  
Although DEMATEL method can calculate the importance degree of a specific prevention method in the influencing factor system, it cannot determine the internal correlation of the prevention method and divide the hierarchy, so it is difficult to effectively manage and control the prevention method. Therefore, this paper uses ISM method to classify the system structure. The ISM method can transform fuzzy ideas into intuitive ones with good structural relations, and is suitable for system analysis with many variables, complex relations, and an unclear structure [47].  
ISM is a recognized method for identifying relationships between specific elements that define a problem. ISM is a mature qualitative tool, which can be applied to various disciplines. For example, [PERSON] et al. through the application of ISM, this paper discusses various obstacles of green supply chain management (GSCM) in Indian automobile industry [50]. [PERSON] et al. apply the ISM method to understand the interaction between total quality management (TQM) barriers in an organization [51]. [PERSON] et al. analyzed the key success factors of world-class manufacturing practices using ISM method [52]. The basic steps to implement ISM are as follows:  
1. Calculate the overall influence matrix F. The calculation formula is: \[\text{F}=\text{I}+T=\left[f_{ij}\right]_{n\times n}\] (9) where, the matrix I is the identity matrix;
2. The threshold is introduced to eliminate redundant information for the most streamlined matrix. According to the trial calculation, the most suitable threshold calculation model is obtained. The calculation formula is: \[\lambda=\alpha+\beta\] (10) In this equation, \(\alpha\), \(\beta\) are the mean and standard deviation of all the elements in T.  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Linguistic Variables** & **Triangular Fuzzy Number (TFN)** \\ \hline N (No influence) & (0,0,0.2) \\ VL (Very low influence) & (0,0.2,0.4) \\ L (Low influence) & (0.2,0.4,0.6) \\ H (High influence) & (0.4,0.6,0.8) \\ VH (Very high influence) & (0.8,1,1) \\ \hline \hline \end{tabular}
\end{table}
Table 3: Semantic transformation table.  
According to the overall influence matrix of the system and the threshold value to remove the redundant factors, the reachable matrix M is obtained.  
\[m_{ij}=\{\begin{array}{l}1,f_{ij}\geq\lambda\\ 0,f_{ij}<\lambda\end{array}(i,j=1,2,3,\cdots,n) \tag{11}\]  
In formula, 1 means there is a direct effect between the two factors, while 0 means there is no direct effect between the two factors.
4. The accessible set and the preceding item set of each factor were determined, and the accessible set Ri and the preceding item set Si were obtained by hierarchical processing. \[\begin{array}{l}\text{Ri}=\left\{\text{a}_{\text{j}}|\text{a}_{\text{j}} \in\text{A},\text{k}_{\text{ij}}\
eq 0\right\},(\text{i}=1,\cdots,\text{m})\\ \text{Si}=\left\{\text{a}_{\text{j}}|\text{a}_{\text{j}}\in\text{A},\text{k}_{ \text{ij}}\
eq 0\right\},(\text{i}=1,\cdots,\text{m})\end{array}\] (12)
5. Check the following. If it is true, it indicates that the corresponding factor is the underlying factor, and then the rows and columns corresponding to the factors are deleted in the matrix M. \[\text{R}_{i}=\text{R}_{i}\cap\text{S}_{i}\] (13)
6. Repeat step 4 and step 5 until you get the factor set Nq (q = 1, 2,..., n) at each level and all factors in the reachable matrix M are deleted.
7. According to the matrix obtained in step 6, the hierarchical structure diagram of influencing factors is drawn in the order in which the factors are crossed out.