ETF智投系列研究之十二：上半年债券ETF成吸金主力品种

The provided content does not include any quantitative models or factors, nor does it discuss their construction, evaluation, or backtesting results. The report primarily focuses on the growth and trends in the ETF market, including the performance of various ETF categories, fund company strategies, and fee adjustments. No quantitative models or factors are mentioned in the provided documents.