Spread duration is not what you suggest. Standard duration measures sensitivity to parallel shifts in the spot curve. Spread duration measures sensitivity to parallel shifts in an instrument's (typically option-adjusted) spread over Treasuries. It is widely used with floatering rate notes. Suppose you have a floater that was originally issued for seven years with a floating coupon of 6-month Libor plus 250 basis points. Now it is a year later. The issuer's notes are still trading at 250 over, but you are wondering what would happen to the value of the floater if that spread dropped to 150. Spread duration answers that question. The concept isn't so relevant for other instruments. For example, the spread duation of a fixed-coupon, non-callable corporate bond is simply its duration.

我又仔细研究了一下这句话，这句话的背景是在讨论计算Portfolio利差久期时，对包含Treasury和不包含Treasury的Portfolio有什么不同。
P19页EXHIBIT5中可以看到，计算一个Portfolio的SpreadDuration，只把其中非Treasury的Sector的SpreadDuration加权求和，而每个Sector的Spread Duration是假设和Duration是相等的。Treaury的Spread Duration等于零。有了这些，你在看看书上这句话就比较好理解了吧？

xia xia !!!

又学到了。。。赞赞