PDA sales have continued to drop at the expense of smart phones, according to the latest figures from IDC.

Worldwide shipments of PDAs - defined as pocket-size devices that synchronize with PCs but don't include voice capabilities - fell 21 percent to 1.7 million units in the second quarter of this year compared to last year. It is the sixth straight quarter fall.

Meanwhile, smart phones or data-enabled devices like PalmOne's Treo 650, Nokia's 6670 and RIM's BlackBerry have continued to grow. Smart phones give users the ability to download mobile applications, browse the Internet and check e-mail in addition to making phone calls, leaving PDAs looking for a market.

Analysts have been chronicling the decline of the traditional PDA for years. Modern mobile phones can often perform all of the traditional personal information management functions like storing contacts and calendar appointments, at a fraction of the cost. Sony and Toshiba have even stopped selling their devices worldwide, citing the drop-off in demand.

PalmOne remains top of the PDA market, but its shipments fell 31 percent compared to last year. HP comes second but saw a decline of 39 percent. However, both companies are developing hybrid phone/PDA devices that are expected to form the backbone of their product lineups for years to come.

Acer surged into third place with a 600 percent increase in shipments - the result of demand for its n35, which comes with an integrated GPS receiver. This shows that innovative PDA designs can still attract users, IDC said.

Shipments of Dell's Axim PDAs were up 17 percent compared to last year, good enough to capture fourth place among PDA vendors. IDC attributed Dell's performance to its PC market leadership, which allows it to bundle Axim PDAs with PC sales.