That's why it really depends on how you look at your lost. Going back to 15% lose, I would say that its not a good thing to sell because we all know that the market is very volatile and a swing of 15% regain is very common. So it really depends on a lot of factor specially how you look at your investments, whether short or long, just saying.
Disastrous for those going short. 15% is painful already, what more for an 80% negative. Though i doubt many could've expected how the market plunged to these levels recently. Most of us are expecting a recovery instead of this.