Investors might want to bet on Groupon (GRPN) , as it has been recently upgraded to a Zacks Rank #1 (Strong Buy). This rating change essentially reflects an upward trend in earnings estimates -- one of the most powerful forces impacting stock prices.The sole determinant of the Zacks rating is a company's changing earnings picture. The Zacks Consensus Estimate -- the consensus of EPS estimates from the sell-side analysts covering the stock -- for the current and following years is tracked by the system.The p ...