One of England’s grand houses that served as a hospice for wounded troops in the Great War has an oak-paneled library where one of the panels is speckled with small holes drilled by stray darts hurled wide of a dartboard. The statistical chance of hitting the bullseye on that wall was greatly reduced by whatever caused so many stray throws… like the distance, the light, and the condition of the players. A statistical bell curve can be built that represents variance from the averaged throws. Within the central one-third of the bell curve, where most of the “hits” are concentrated, the formula quantifies the deviation from the performance mean for 67% of total hits below the curve… the Standard Deviation. It is common practice to base our judgements upon the central third of all the scores, and treat the rest (in this case, the holes in the wall) as less-informative, or even insignificant, outliers. The formula for determining Standard Deviation can also be applied to describe the range of performance volatility arising from your particular mix of stocks, bonds, and cash; it can provide insight concerning the extent of your market risk. From another perspective, if you can pinpoint your tolerance for market risk, you can use Standard Deviation as a guide for assembling assets in your portfolio. An experienced advisor can help you do that without leaving any tiny holes in your wall.
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