Today: December job gains take a steep dive from previous months, but economists doubt Federal Reserve will halt changes to bond purchases. Also: Silicon Valley tech stocks gain as PC companies avoid declines after worst year on record.

The Lead: Job gains plummet, but few think Fed will change course

The United States added 74,000 jobs in December, a steep drop after four straight months with totals higher than 200,000, a result that left economists befuddled but is unlikely to immediately change the Federal Reserve's view that the economy is strengthening enough to pull back on stimulus efforts -- for now.

The smallest monthly job gain in three years follows a strong fall and winter for the economy, which convinced the Fed to begin tapering its monthly $85 billion bond purchases. The unemployment rate did plunge from 7 percent to 6.7 percent for the month, though that was credited to potential workers giving up the search for a job, with the "participation rate" falling to 62.8 percent, only the second time the percentage of people in the workforce declined that low since 1978.

The easiest explanation for December's numbers was the severe cold that infected most of the country, which could have cut back the number of people willing to look for work as well as the potential for hiring.

"The weather probably did play a big role," JPMorgan Chase chief U.S. economist Michael Feroli told Bloomberg News. "It's a reminder that the improvement is not going to be a straight line," he added.

The stark decline cannot all be attributed to the weather, though, Morgan Stanley economist Ted Wieseman countered: the cold snap likely only affected 50,000 to 75,000 jobs, which would still leave December much lower than previous months, which were revised to add even more new jobs in Friday's Labor Department report.

"Weather was an important contributor to the softness in December payroll job growth but not enough to explain all of the softness," Wieseman wrote in a note.

Aside from weather, experts seemed unable to explain the odd result, with many noting that the number could be revised upward in the January report.

"If there ever there was a curveball, this was it. These limp numbers are as puzzling as they are surprising," MB Capital trading director Marcus Bullus told Reuters.

"There is a good possibility this is just a one-shot deal that could either get revised away or made up for in next month's release," Bank of the West chief economist Scott Anderson wrote in a note.

For that reason, few think that the result will end up changing the Fed's course when the central bankers meet later this month. Writing for The Wall Street Journal, Jon Hilsenrath said that a single weak report with possible weather effects would not change the broader, positive economic picture created in the previous months.

"That said, Friday's report should put to rest for the time being any notion that the Fed will reduce the bond-buying program more quickly than planned," Hilsenrath wrote.

SV150 market report: Tech stocks gain as PC companies gain despite reports

Another indication that December's jobs report was widely viewed as a blip on the economic radar was Wall Street's reaction: The broad-based Standard & Poor's 500 gained 0.2 percent on the day, and Silicon Valley stocks gained more as personal-computer companies gained despite a poor 2013.

Hewlett-Packard (HPQ) increased 0.3 percent to $27.70 after research firms reported Thursday afternoon that it had officially lost its title as largest PC manufacturer during the worst year on record for the industry. Other companies deeply invested in the PC industry also avoided a day-after letdown: Intel (INTC) advanced 0.9 percent to $25.53 while RBC Capital wondered about the chipmaker's mobile strategy, and Sunnyvale rival Advanced Micro Devices gained 2 percent to $4.17. Microsoft even received an upgrade from Barclays analyst Raimo Lenschow before adding 1.4 percent to $36.04.

One PC manufacturer about which Gartner and IDC disagreed did fall Friday -- Apple (AAPL) declined 0.7 percent to $532.94 even while clocking a win in its patent battle with Google (GOOG)-owned Motorola Mobility. Google barely moved Friday, losing 6 cents to $1,130.18 after being forced to apologize for an embarrassing Hitler-related error on Google Maps; the Mountain View search giant may have a new trick up its sleeve, though, as Bloomberg News reported that employees from the Google X division met with the FDA, which regulates medical devices.

Tesla Motors (TSLA) declined 1.2 percent to $145.72 after updating software and its wall chargers to help avoid fires like one in which the Palo Alto company recently denied involvement. Facebook gained 1.3 percent to $57.94 after an upgrade from Stifel Nicolas, which likes the Menlo Park company's plans for Instagram and video advertisements, but does face a new lawsuit stemming from its News Feed advertisements. Rival Twitter closed out a rough week on Wall Street with just a slight decline of 0.1 percent to $57, while Campbell security company Barracuda Networks -- which went public only one day ahead of Twitter -- dropped 6.5 percent to $36.47 despite living up to expectations in its first earnings report as a public company.

Up: Yelp, Juniper, Pandora, AMD, LinkedIn, Facebook, Oracle (ORCL), Salesforce, Electronic Arts (ERTS), Workday, Splunk, Intel, Gilead, Yahoo (YHOO), Adobe (ADBE), Cisco (CSCO), VMware, Applied Materials, HP

Down: Netflix (NFLX), Tesla, SunPower (SPWRA), SolarCity, NetApp, Apple, Intuit (INTU), Ruckus, Symantec, Nvidia, Twitter

The SV150 index of Silicon Valley's largest tech companies: Up 5.55, or 0.37 percent, to 1,497.07

The tech-heavy Nasdaq composite index: Up 18.47, or 0.44 percent, to 4,174.66

The blue chip Dow Jones industrial average: Down 7.71, or 0.05 percent, to 16,437.05

And the widely watched Standard & Poor's 500 index: Up 4.24, or 0.23 percent, to 1,842.37

Check in weekday afternoons for the 60-Second Business Break, a summary of news from Mercury News staff writers, The Associated Press, Bloomberg News and other wire services. Contact Jeremy C. Owens at 408-920-5876; follow him at Twitter.com/jowens510.