The most authoritative and directly relevant evidence comes from a reputable Korean economic news outlet that describes recent KOSPI movements during a period of global market volatility. The article indicates that the KOSPI recently surged past 6300 after a volatile period, implying that the index experienced noticeable fluctuations before recovery. This supports the notion that the KOSPI had previously declined during the described volatility, though the exact magnitude of a 10% drop is not confirmed by direct numerical evidence. Other sources, including brokerage press releases and individual stock data on Yahoo Finance, provide indirect context but do not contradict the possibility of a temporary 10% index decline. While the main source lends credibility to the statement, the absence of exact quantitative confirmation limits full certainty, leading to a medium confidence assessment that the KOSPI likely decreased by around 10% amid global volatility.