iTap (from Lexicus, Motorola) and T9 (Tegic) use different strategies for predicting words from the pressed keys.

1. T9 is used on Nokia and on many other brands of phone. iTap on Motorola only(?).

2. According to two researchers from Motorola `iTAP is better'

3. iTap offers word-completions. (In my opinion, this feature, while nice for long words, makes iTap harder to explain, since a novice user, having heard that iTap does word completions, is likely to be demoralized and confused by the bad predictions that are made when only half the word is written. When I explain T9 to people I tell them to ignore the display until they have finished the word.)

4. ITap's predictions are context-dependent. This means it can predict whole sentences, which is nice, if you are a predictable writer. But T9 advocates would emphasize the advantage of T9's being NOT context-dependent is that you know that to write a particular word, you can memorize a particular key sequence - for example, to write "HOME", you always press "4663**" (or some such), independent of context. This is good for useability, as it means the experienced user can go fast and doesn't need to look at the display.

5. From this Motorola review: `iTap has its faults. For one, pressing the 1 button defaults to putting the number 1 in the word instead of putting a period. If you enter a space after the word, the 1 key will default to a period, but not if you are at the end of a word. This is real annoying, as you either have to waste a character at the end of each sentence, or you need to waste a keystroke to select the period instead of the 1. What were they thinking?'

6. You can correct iTap as you write a word, and `lock in' your corrections, by using the arrow buttons. (This option is not available in T9 - and perhaps for good reason, since it is often not necessary to make corrections.) The recommended way of adding a word to iTap's dictionary is to use this `lock in corrections as needed' approach, rather than the simple `multitap' (abc) approach chosen in T9. This means that in iTap, you have to keep switching buttons (from 1-9 to the arrow buttons)

7. In T9, '0' is used to insert a space (and implicitly to confirm that the displayed word is fine). In iTap, 'Select' is used to terminate words AND to insert a space. Pressing Select twice, in iTap, will send the text message.

8. You can enter symbols and numbers in iTap without switching mode.

9. My take on the difference between iTap and T9: T9 is very simple to explain: iTap has more features which make it harder to explain, and perhaps it demands a little more attention from the user too. iTap makes the user make decisions of the form `shall I stop writing the word now, and try to find it in the word completion mode, or shall I continue writing the word?'

A user faced by such choices may find he regrets his decisions. T9 doesn't bother the user with such choices. You just keep going, and you'll be writing at close to one character per key press, which is fine. I never regret using T9.
Users may also misunderstand the choices they are offered in itap: they may think that, since they are offered the chance to correct the word on the fly as they write it, they should do so; but doing so leads to slower writing.

External links

Some notes on iTap