Two research firms on Wednesday said growth in computer shipments worldwide will be more robust than they had previously forecast for the year, driven by continuing strength in sales of portable computers. IDC now expects shipments to rise 15.2 percent over last year, compared with a 2008 forecast of 12.8 percent growth that the Framingham, Mass.-based firm made in March. Stamford, Conn.-based Gartner, which uses different methods than IDC to track shipments, now forecasts a 12.5 percent increase from the 264 million units it counted last year, up from its 10.9 percent projection three months ago.

IDC's new forecast predicts global PC shipments will reach 310 million units this year, up from 269 million last year, when the growth rate was about 14 percent compared with 2006.

Portable computers have posted stronger growth rates than desktop models in recent years, and IDC projects a 35 percent growth rate for portable models this year. Those gains have recently offset slow growth in U.S. demand for PCs amid an economic slump.

Factors driving recent portable PC growth include the replacement of desktop models with notebook computers and the emergence of extremely low-cost portables popular in developing nations.

"Despite recent economic pressure, the consistent gains fueled by portable adoption, falling prices, and new users - particularly in emerging regions - will continue to drive growth during the forecast," said Loren Loverde, director of IDC's worldwide quarterly PC tracker.

Low-cost portables such as those built off Intel Corp. s Classmate PC platform and the "XO" from the One Laptop Per Child initiative had previously been excluded from IDC's statistical tracking because of their nontraditional designs and features such as relatively low processing power and storage capacity. But IDC said that it has now begun counting such models because "the latest versions of these systems are now more robust, meeting IDC's criteria to be considered PCs, and actual shipment volumes are rising."

Because the ultra low-cost PCs account for less than 1 percent of the overall PC market, the decision to begin counting those PCs was a smaller factor in Wednesday's higher forecast than the overall shipment growth for portables of all types, Loverde said. IDC projects 3.5 million ultra low-cost PCs will he shipped this year.

Portables generally cost more than desktops, and portables' increasing share of the overall market is expected to help offset falling average computer prices, IDC said. The firm projects the total value of PC shipments to grow by 9.6 percent this year to $286 billion, with shipment value growing by 5 percent to 6 percent per year from 2009 through 2012.

IDC projects shipments of all PCs to grow at double-digit rates in percentage terms in 2009 and 2010, before slowing to growth in the high single-digits through 2012, when shipments are projected to total more than 472 million.