This is a survey. What percentage of your stock index trades are winners, and what's your average win amount vs. average loss amount per contract. All strategies, from scalping to long term are welcomed to participate.
70% and above seems to be the minimum threshold level to be successful short term traders (hours to a few trading days). Though traders hitting that win% tend to have average win smaller than average loss. If you're really good, you'll hit >70% win, and about equal or greater win amount to loss amount.