Effectively! deviations bigger than 4 times of what one would expect.
Just for my own information: how many hours/days do you usually hold your index spread?
On these 2 occasions ( august 2015, april 2016) : how long did you stretch your holding period?
and as you averaged, how much bigger was your final ( or maximal) clip size compared to average?
When I used to do it, I would hold between 15 minutes (good times) up to 14 hrs (bad times). On that august period, I exited 10-minutes before the close which was U.S. Afternoon. Sometimes I'd have to hold into the Aussie re-open and exit in the auction. But then this got dangerous.. really dangerous... so that's a story for another time.
In terms of averaging, I would always do a smaller clip on the second one. So first clip was 1, next was half, then next was half again. On really tempting days, I would go half and half again and have 3 clips. But the probability of winning has significantly changed.
I don't do the index spread anymore nor bond spreads until they behave. I am doing outrights and trading off price action. The risk/reward is a lot better and I can transfer my skills across products.