資産価値の節目下回る確率と特定リスク

最近リスクラッテの問題を解こうとして苦しんでいる。
Given the mean and volatility of a diffusion process, the reader is asked to determine the probability that the asset value never drops to the level of 90% of the original value over a ten-year period.
One other problem involves the "specific risk" metric which constitutes the denominator of the Information Ratio. The formula in Alexander and Sheedy's text is: sigmae = sigma(1-p^2), but it is unclear what p alludes to.