A repeat rate that is too fast produces extra characters when the user wanted one

What it is

The intent is one tap, but the finger stays on the key tens of milliseconds extra. If initial delay is short and rate is fast, a second or even third copy of the same character appears. This is not two keys, and not a dropped key; typematic treated a hold as repeat. Older adults, people with unstable motor control, and slow-return membrane boards hit it more. Extra characters cluster on sentence-final punctuation, backspace, and space—keys more often leaned on.

Why it happens

Tap down–up intervals have a distribution. The right tail beyond Tdelay starts repeat. A short Tperiod can squeeze in yet another character before the finger rises. Slow key rebound lengthens down time. Touch has no mechanical rebound and a larger contact, so systems often use a long-press menu instead of character repeat; if both run, a menu and a stream of letters appear together. Software cannot tell from the character whether it was surplus; undo rolls back a chunk, and people think their hand twitched.

Studying it

Measure the target population's tap-duration distribution against the default Tdelay, and compute the fraction of the right tail that exceeds the delay. Then transcribe at that setting and score doubled characters by key class (letter / space / backspace / punctuation). Lengthening delay should drop those errors if the mechanism is right. Do not mix this with contact bounce: bounce is mechanical make-break chatter; repeat is a software timer.

Where it stops holding

Expert typists' taps are short; default delay rarely misfires, and they complain that delay is too long. Games that hold-to-move need immediate repeat or analog values; the “single character” goal does not apply. Sticky Keys and Slow Keys change effective delay. Remote desktop's extra latency may reduce accidental repeat while dulling intentional holds.

Applying it

• Default Tdelay above a high percentile of the target population's tap duration, not to a game-feel short value.
• Backspace and space may use a longer delay of their own, because a mistaken repeat is more destructive.
• Offer policies such as “character repeat off, arrow repeat on.”
• Verify with people whose motor control is slower typing a short sentence, scoring accidental doubles; raising delay should drop that class while an intentional hold-to-delete-a-line remains acceptable in time.

Related

• Same group: C6.31.1 Holding a character key starts repeat; repeat rate and initial delay are independent parameters · C6.31.3 Fast multi-key presses on a keyboard with weak anti-ghosting drop some keys · C6.31.4 N-key rollover reports any simultaneous combination correctly; games and pro boards rely on it
• Adjacent: C6.02 Key travel and tactile confirmation · C6.20 Modifiers and key combinations
• Search: accidental repeat · typematic errors · key hold time