Ten companies have contributed almost half of the gains for the entire S&P 500 index since Oct. 12. Those stocks returned an average of 20% during that time period. One of them, Nvidia, has not yet reported third-quarter earnings. The other nine beat analyst estimates by only 5.6% and had an average one-day return post-earnings of 3.2%. Three of those stocks, AMD, Apple and Amazon, had negative one-day returns post-earnings.