Increase the required interest rate and use expected dividends. Expected dividends may be increasing or decreasing instead of constant. There are more complicated present value formulas. People don't agree on expected dividends. Many people are employed as analysts who carefully read company reports.
Any news can cause stocks and the market as a whole to change in value.
Morris, Kenneth M. and Alan M. Siegel, T he Wall Street Journal Guide to Understanding Money and Investing . Lightbulb Press, 1993. Many different links to company and academic websites. The Nasdaq Stock Market Contains extensive data on stock prices and news reports. Filings with the SEC FreeEdgar gathers all financial reports filed with the Securities and Exchange Commission in a very accessible data base.