Analyst upgrades and revised price targets are strong indicators that can influence investor sentiment and stock prices in the short term. Stifel's upgrade from Hold to Buy reflects a positive change in the firm's outlook on Alector, which can lead to increased investor interest and buying activity. The substantial increase in the price target from $8 to $15 suggests that Stifel sees significant upside potential for Alector, which may result in a short-term price increase as the market reacts to the new information.