# Classification and Causes Identification of Chinese Civil Aviation Incident Reports
## 4 Cause Extraction
### System Design
Here, we provide the details of how we build the keywords dictionary and rules set. Keywords and rules are equivalent to the components of a sentence and grammar, respectively. In modern Chinese, the essential components, such as subject, verb, object, and the order of these components, are consistent in formal paperwork. Thus, three kinds of keywords were sought: the subject keyword (SK), attribute keyword (AK), and evaluation keyword (EK). The SK is the unit responsible for the cause, and it is always the subject in every sentence. Since the reports are written by multiple entities, the same subject may have several representations. For example, more than ten nouns could represent pilots in this context, such as \" \" (Pilots), PF (Pilot Flying), and \" \" (Cockpit), although their independent meanings are not exactly the same. Since H1 to H5 are all descriptions of pilot errors, they share the same SK list. AK is a property or action of SK, which is associated with a cause. Similarly, a single cause can lead to different behaviors in different situations, so each cause has dozens of AKs to be detected. For example, H1 corresponds to \" (estimate/estimation), \" \" (judge/judgement), \" (decide/decision) and so on. The SK and AK can only describe the behavior of a unit, which is not sufficient to make a judgement regarding it, so the EK is introduced. As assessments of the AK, EKs are adjectives and adverbs, corresponding to different parts of speech of AKs. Since they were all negative words, the EKs were only divided into adjectives and adverbs. For instance, \" \" (not good enough), \" \" (be short of), \" \" (too much), \" \" (too low) and \" \" \" \" \" (not proper) were the most frequent adjectives and adverbs among EKs. Specific keywords alone are not sufficient to determine the cause, and rules are needed to guide the usage of keywords. The principle of the rules, as mentioned previously, is to determine one cause from the keywords and other information in the sentence. However, not all cases were detected with rules, and this depended on the type of cause and the written patterns. Six causes (E1, E3-E7) could be determined by only detecting keywords (SKs or AKs). Taking E1 (bird strike) as an example, the occurrence of bird-related words, such as \" \" (bird strike), \" \" (feather) and \" \" (bird's blood), was enough for identification. In identifying these six causes, some cases also adopted special rules. Bird-related words (SKs) and trace-related words (AKs) with no human-related words (SKs)in the context were also needed when identifying E1. In addition, the numbers recorded quantifiable parameters, such as visibility and lateral wind speed. Such values reached a threshold that triggered identification. In detail, visibility below 1,000 meters triggered E3, and a lateral wind speed over 10 m/s triggered E4. For EI-type causes, the occurrence of keyword (SK and AK) combinations could be used for identification. These causes were detected using identical words, although these words were not always in the same order. For example, \" \" (chain break) and \" \" (broken chain) could both trigger the identification of EI2 by detecting \" \" (chain) and \" \" (break). H-type and O-type cause-related rules were more complex because more than one AK and SK were distributed in different clauses of the same sentence. The key was pairing each AK with its corresponding SK. [PERSON] et al. [5] used fixed radii to link keywords, of which the distances from each other less than 7 could be related to each other. As this value could vary for different contents, it is not robust enough, especially in a sentence that contains multiple clauses. Instead of fixed radii, we adopted another effective distance, which depended on other information from the sentence. The scope was initialized with punctuation. Assuming that a clause was the minimal scope, commas and periods could separate clauses from each other. In addition, words like \" \" (and) or \" \" (he) would extend the scope of the SK to next clause.