  Eye typing interfaces enable a person to enter text into an interface using
only their own eyes. But despite the inherent advantages of touchless operation
and intuitive design, such eye-typing interfaces often suffer from slow typing
speeds, resulting in slow words per minute (WPM) counts. In this study, we add
word and letter prediction to the eye-typing interface and investigate users'
typing performance as well as their subjective experience while using the
interface. In experiment 1, we compared three typing interfaces with letter
prediction (LP), letter+word prediction (L+WP), and no prediction (NoP),
respectively. We found that the interface with L+WP achieved the highest
average text entry speed (5.48 WPM), followed by the interface with LP (3.42
WPM), and the interface with NoP (3.39 WPM). Participants were able to quickly
understand the procedural design for word prediction and perceived this
function as very helpful. Compared to LP and NoP, participants needed more time
to familiarize themselves with L+WP in order to reach a plateau regarding text
entry speed. Experiment 2 explored training effects in L+WP interfaces. Two
moving speeds were implemented: slow (6.4{\deg}/s same speed as in experiment
1) and fast (10{\deg}/s). The study employed a mixed experimental design,
incorporating moving speeds as a between-subjects factor, to evaluate its
influence on typing performance throughout 10 consecutive training sessions.
The results showed that the typing speed reached 6.17 WPM for the slow group
and 7.35 WPM for the fast group after practice. Overall, the two experiments
show that adding letter and word prediction to eye-typing interfaces increases
typing speeds. We also find that more extended training is required to achieve
these high typing speeds.
