The virtual Qwerty keyboard on Apple's iPhone allows users to enter text as quickly as they would on another handset's physical keyboard, but they'll make rather more mistakes in the process.

That's the conclusion drawn by a Chicago usability consultancy after watching 60 punters tap away on a variety of handsets. Twenty of them used iPhones, another 20 used full-size BlackBerries, and 20 more were given Samsung E300 numeric pad-only phones to try.

Each triallist was told to type out six fixed-length text messages, while iPhone users also had to enter two sentences containing all the English letters, along with a block of text that contains the letters in the frequencies they most commonly appear in in written work.

The text messages were constructed to prevent the quirks of numeric-pad text entry favouring or hindering users. Curiously, the use of predictive text was not part of the test.

Accordingly to the surveyor, User Centric, the BlackBerry users punched out their missives as quickly as the iPhone users did. However, the latter group made, on average, 5.6 text-entry errors per message, to the BlackBerry team's 2.1 errors per message. They were just ahead of the Samsung group, who scored 2.4 errors per message.

As a control, User Centric also tried non-iPhone owners out on handsets they were unfamiliar with, each individual typing out six more fixed-length messages. People who've primarily used numeric pads for texting made fewer mistakes when they moved on to a physical Qwerty pad than they did on the iPhone's touchsensitive screen. And they were faster on the hard keyboard than the virtual one.

Numeric-pad phone owners made an average of 5.4 errors per message on the iPhone, 1.2 errors per message on the physical Qwerty phone and 1.4 errors per message on their own phone.

"Participants also indicated a preference for hard-key Qwerty phones when texting," said User Centric's Jen Allen.

But what of the iPhone's much-vaunted ability to monitor users' input and adjust itself accordingly for better typing results?

Thumbs-down, alas: "While the iPhone's corrective text feature helps, this data suggests that iPhone users who have owned the device for a month still make about the same number of errors as the day they got it," User Centric's Managing Director, Gavin Lew, said.

You can read User Centric's write-up of the test on its website.