These are mostly driven by subjective factors that are hard to see and measure in real time.Therefore, the Zacks rating upgrade for Bancolombia basically reflects positivity about its earnings outlook that could translate into buying pressure and an increase in its stock price.Most Powerful Force Impacting Stock PricesThe change in a company's future earnings potential, as reflected in earnings estimate revisions, has proven to be strongly correlated with the near-term price movement of its stock. The influence of institutional investors has a partial contribution to this relationship, as these big professionals use earnings and earnings estimates to calculate the fair value of a company's shares. An increase or decrease in earnings estimates in their valuation models simply results in higher or lower fair value for a stock, and institutional investors typically buy or sell it.