但是信贷机构争辩说他们的评级只能衡量违约率的可能性，而并不对个别的证券提出买进的建议，也不能预测市场价格。
But the agencies argue that their ratings are designed to measure the probability of default, not to recommend the purchase of individual securities or to predict market prices.
它们的数学模型依赖于过去的交易规律，在其它证券价格变化(例如下跌)时，预测某种证券未来的表现。
Their mathematical models rely on past trading patterns to predict how particular securities will perform in the future if other securities, say, fall in price.
因此，构建组合预测模型来预测证券价格的波动，既具有一定的理论价值又具有较强的现实指导意义。
As a result, constructing the model of combined forecasting to predict the fluctuation of stock price has a theoretical value and a strong guidance.
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