ITR Atlas
← All interfaces

Mobile Touchscreen Typing (Apple, 2007)

Measured by Palin, Feit, Kim, Kristensson & Oulasvirta · ACM MobileHCI 2019 (2019)

Touch Keyboard 2007

Inputs

The measured or assumed values behind the calculations, each with its source.

rate = 36.2 wpm
Average typing speed across 37,370 volunteers (27M keystrokes). The system date is the 2007 iPhone full-QWERTY touch keyboard; the throughput is measured in the 2019 mobile typing study.
P = 0.9766
1 − 2.34% average error rate (uncorrected error rate reported in the study)
H = 1.0 bits/char
English-text entropy (Shannon).
N = 30
Distinguishable keys on the on-screen QWERTY, used only for the raw-key Wolpaw ceiling (uniform prior). The headline Shannon figure uses English entropy instead, since touch keystrokes with autocorrect are far from uniform.
T_key = 0.3315 s/keystroke
Gross keystroke interval for the Wolpaw ceiling: 60 / (36.2 wpm × 5) = 0.3315 s. Gross because Wolpaw's accuracy term P handles errors separately.

Strictest ITR

Each scoring method is an upper bound on the channel, so the headline is the strictest (smallest) one for this entry. Use the score selector on the home page to view any single method across entries.

Strictest Shannon (text) Recomputed
Character-entropy throughput
Net of English redundancy and typing errors
2.95 bits/s

  1. Net words per minute

    36.2 wpm × (1 − 0.0234) = 35.3 net wpm

  2. Characters per minute

    35.3 × 5 chars/word = 176.7 chars/min

  3. Information transfer rate

    176.7 char/min × 1.0 bit/char ÷ 60 s/min = 2.95 bits/s

What counts as a bit depends on the action space. The number of distinguishable actions and how likely each one is are design choices of the task, not the sensing hardware. The same modality can present a fixed set of targets, a set pruned per step by a grammar or language model, or a continuous control space. Each of these changes how many actions are live and how the probability mass is spread, and therefore the information per selection. Read the action space below before comparing headline numbers across entries.

Action space

What the user can produce at each step, and how those options are distributed.

Structure
Fixed set of targets
Size
30 distinguishable actions
Prior
Context-conditioned: likelihoods depend on prior actions
Notes
On-screen QWERTY with autocorrect and word prediction in the loop; the study found autocorrect raises entry rates. The language model makes the prior context-conditioned, so this is not a uniform key-selection figure.

Comparability The strictest bound here is the Shannon entropy of the output text, under one predictor held constant across the whole atlas (≈1 bit per character). That shared predictor makes it directly comparable to every other text entry (keyboards, spellers, silent speech and speech BCIs) regardless of prior or vocabulary size. For most text interfaces it comes out tighter than the raw-selection bounds, but not always. Where a small vocabulary makes Wolpaw tighter, that wins instead. Any Fitts, Wolpaw or log₂(N) figure shown below is another bound on the same channel. Switch the home-page score selector to compare one across entries.

Other bounds considered for the headline

Also valid upper bounds for this entry and eligible to be the headline. They just came out looser than the strictest above. Pick any of these in the home-page score selector.

Wolpaw Recomputed
Wolpaw bitrate over the raw key set
Uniform-prior ceiling on the touch-key channel, before English redundancy
14 bits/s

  1. Bits per selection (Wolpaw formula)

    B = log2(N) + P*log2(P) + (1-P)*log2((1-P)/(N-1))
  = log2(30) + 0.9766*log2(0.9766) + 0.0234*log2(0.0234/29)
  = 4.633 bits / selection

    Term 1 is the information if every choice were correct; terms 2-3 subtract the bits lost to the error rate, assumed spread evenly over the other N-1 targets.

  2. Selections per second

    T = 0.3315 s/selection  ->  1 / 0.3315 = 3.017 selections/s

  3. Information transfer rate

    ITR = B * selections/s = 4.633 * 3.017 = 13.976 bits/s

Other score types

Bounds the atlas keeps out of the default strictest headline: as-reported figures, alternate task conditions, or raw-channel ceilings that shouldn't win the headline by default. Each still carries a score type, so the home-page selector ranks this entry on it when you choose that type. Read its derivation before comparing across entries.

Nuyujukian Recomputed
Nuyujukian achieved bitrate over the raw key set
Achieved-bitrate view of the touch-key channel, shown for comparison
14 bits/s

  1. Achieved-bitrate credit per net-correct key

    N = 30 keys → log2(N − 1) = log2(29) = 4.86 bits per net-correct selection (field-standard achieved bitrate, e.g. Webgrid; Nuyujukian 2015, which introduced the metric, used log2(N)).

  2. Net-correct key rate

    net-correct = 2P − 1 = 2(0.9766) − 1 = 0.953 of keys. At 0.3315 s/key → 0.953 / 0.3315 = 2.87 correct/s.

    A touch error commits the wrong character rather than timing out, so incorrect = 1 − P. Same N (30 keys), uncorrected accuracy (97.66%) and keystroke interval (0.3315 s) as the entry's raw-key Wolpaw ceiling, and lands on the same ~14 bits/s. Both are the uniform-prior key channel before English redundancy, above the 2.95 bits/s Shannon headline.

  3. Achieved bitrate

    4.86 bits × 2.87 correct/s = 14.0 bits/s.

Source

Authors
Palin, Feit, Kim, Kristensson & Oulasvirta
Publication
ACM MobileHCI 2019, 2019
Paper
10.1145/3338286.3340120
Reference
System date: Apple 2007 iPhone announcement with full-QWERTY touchscreen keyboard