Although we don't believe in timing the market or panicking over daily movements, we do like to keep an eye on market changes -- just in case they're material to our investing thesis.

Six in 10 stocks rose on Thursday, with eight of the 10 major market sectors also advancing in reaction to Federal Reserve Chairwoman Janet Yellen's Capitol Hill testimony this morning. Yellen, who became the first female to chair the Fed more than a century after the central bank's foundation, stuck to the "tapering" process predecessor Ben Bernanke began implementing in December. In other words, the Fed is slowly weaning the financial system from its all-powerful teat of liquidity, and in doing so asserts its belief in the economy's ability to survive without the love of its sugar mama. The S&P 500 Index (^GSPC 0.03%) added 9 points, or 0.5%, to end at 1,854. 

While it's flattering that the Federal Reserve doesn't think the U.S. financial system will wither and die without its assistance, a mere expression of confidence isn't quite enough to shore up a balance sheet or swing a company to a profit. No matter how brilliant your mother thinks you are, if your lava doesn't flow on cue at the science fair, the judges won't shower you with compliments. The proverbial lava wasn't flowing on cue for oil refiners today, and each of the S&P 500's three worst performers hailed from the beleaguered industry.

Shares of Marathon Petroleum (MPC 1.07%) tumbled 4.4% Thursday, as the difference between crude oil prices in the U.S. and Europe shrank. U.S. refiners make a living by exploiting the price difference in the two markets, buying crude cheaply in one region, refining it into a more usable medium like gasoline, and selling the refined product globally. It's not Marathon Petroleum's fault that, to reprise the metaphor, its lava didn't flow on cue, but the judge (the stock market, in this case) only cares about the results, which don't look attractive today.

Oil and gas stocks were one of only two market sectors to slip on Thursday, and shares of Marathon peer Valero Energy (VLO 1.29%) also ended in the red due to the pressured margins mentioned above. Valero Energy shed 4.4% in trading, and unfortunately swings like today's are nearly unavoidable for investors in this corner of the markets. As the Brent-WTI spread -- the difference between U.S. and European crude oil prices – narrows, so too do Valero's margins.

Another ill-fated refiner, Phillips 66 (PSX 0.33%), found itself as one of the day's laggards for the same reasons. Shares lost 2.6% Thursday in the wake of the declining Brent-WTI spread. Phillips 66 investors can place some of the blame on U.S. crude oil inventories, which advanced far less than expected last week. Expecting an increase of 800,000 barrels in storage, we instead saw just an increase of just 68,000 barrels, sending U.S. crude prices higher.