I see different values for key corporate statistics on different websites (Marketwatch, Yahoo, Finviz, etc). Which source can one trust? Is it possible to get a good read on a company by researching online, or should one ignore the internet and wait for quarterly filings before doing one's own calculations?
Major financial sites and brokerages provide trustworthy data. There are sometimes differences in the way the data is presented. For example, the Price to Earnings ratio can be based on the past 12 months or the projected 12 months in the future. Dividend yield can be past for the year (trailing) or the next year (forward). Income and asset information can be based on a particular quarter or year, so one site may update its data sooner than another, causing a difference.
Probably the most reliable and up-to-date place for a company's data is from the company itself. But there's generally not a problem getting data from any major financial site or brokerage site.