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

Friday ushered in another bizarre case of economic data falling short of expectations; yet, it was a positive for the U.S. investor. Ultimately, it helped push the S&P 500 (SNPINDEX:^GSPC) to a gain for the eighth time in its past nine sessions.

Today's economic data was a mix of somber to poor across the board. The Michigan Consumer Sentiment Index tumbled to 76.8 for September compared to forecasts which had called for a reading around 82. This would mark a sharp decline from the 82.1 reported in August, and could signal a changing attitude among consumers who are seeing signs of weakness in the economy. U.S. retail sales growth in August of 0.2% also lagged estimates, and would confirm the consumer sentiment figures' suspicion that the consumer is more anxious that he or she was last month.

On the bright side, weak retail sales figures, and a worried consumer, could give the Federal Reserve a reason to hold off on tapering its monthly bond-buying program known as QE3. This free money program is largely credited with helping keep interest rates near record lows, and in buoying the housing industry -- and, frankly, investors would love to see it remain in place.

With that being said, the S&P 500 advanced by 4.57 points (0.27%), to close at 1,687.99, less than 22 points away from setting another new all-time closing high.

Leading the pack, yet again, is video game and gaming accessories chain GameStop (NYSE:GME), which added 6.1% on the day despite no company-specific news. No company over the past year has topped the S&P 500 in gains more than GameStop, which has been pushed higher in anticipation of robust sales with the coming debut of Microsoft's Xbox One and Sony's PlayStation 4. Furthermore, the debut of Madden NFL's 25th anniversary game for the current gaming consoles is bound to pump up investor optimism. As for me, I remain concerned that this could be a "buy the rumor, sell the news" type of event. GameStop is certain to see increased sales for the time being with the debut of new gaming consoles, but its share price has already risen dramatically, and the lag time between next-generation console development is incredibly long. In other words, a year from now, GameStop shareholders could be very disappointed with their company facing some steep same-store sales comparisons.

Grocer Safeway (NYSE:SWY) also rose 6.1%, but missed out on being today's top-performing S&P 500 component by a nose (6.07% for GameStop vs. 6.05% for Safeway), following an upgrade by Credit Suisse. The move by Credit Suisse took Safeway all the way from an underperform to an outperform rating, and moved its price target to $34, from $26. Specifically, the covering analyst notes that Safeway's CEO's attempts to improve shareholder value, and suggests that Safeway could unlock shareholder value and improve margins by divesting itself of non-core assets. It's always important to remember that the supermarket industry is based on razor-thin margins, so any way to improve those margins could have a profound impact on profits. Personally, I've appreciated Safeway's attempt to freshen its store image and focus on more organic, high-margin products, and feel the upgrade is long overdue.

Finally, biotech juggernaut Regeneron Pharmaceuticals (NASDAQ:REGN) followed in Safeway's path, and rose 6% after Lazard Capital Markets boosted its price target to $325, from $298, and kept its rating at buy. The covering analyst, Joshua Schimmer, points to a co-developed drug with Sanofi (NYSE:SNY), known as dupilumab, that's currently being tested for allergic asthma and atopic dermatitis as its next big catalyst. With additional indications, Schimmer belives this experimental drug could impact up to a $10-billion market if approved. Given the ongoing dominance of Regeneron's lead drug Eylea, it's hard to argue against its results, but I also feel that much of Regeneron's expectations have already been baked into its share price.

Fool contributor Sean Williams has no material interest in any companies mentioned in this article. You can follow him on CAPS under the screen name TMFUltraLong, track every pick he makes under the screen name TrackUltraLong, and check him out on Twitter, where he goes by the handle @TMFUltraLong.

The Motley Fool owns shares of GameStop, Lazard and Microsoft. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.