1, data reliability: in order to ensure the reliability and validity of the research results, the quality of the data must be guaranteed. Therefore, choosing available data for 5 years can avoid the problem of outdated or invalid data.

2. Particularity of research objects: The particularities of different research objects need at least 5 years of data. For example, if the research object is historical events or phenomena in a specific period, it is necessary to select the data of that period for research.