Quote from m22au:
I *think* I have an answer.
The NAV is set at 7am each day.
At 4pm on Wednesday, silver was at $35.21. ZSL at $21.74
At 7am on Thursday, silver was at $32.66. ZSL range from 24.00 to 24.90.
Therefore silver lost 7.24% during that time, and therefore one would expect ZSL (at 7am Thursday) to be 15.48% higher than the price at 4pm on Wednesday.
21.74 * 1.1548 = 25.10
At 4pm on Thursday, silver was at 34.12. (up 4.47% from 7am)
ZSL was at 21.80.
If we use $24.21 as the price for ZSL at 7am on Thursday, then the 4pm price of 21.80 represents a 9.94% decline (2 * 4.47%).
NAV (at 7am) on Thursday 12 May $24.29
http://www.proshares.com/funds/zsl.html