# Sensors, Techniques, and Future Trends of Human-Engagement-Enabled Applications: A Review
## 4 Approaches  
This section reviews common approaches to human engagement estimation.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & **Cues** & **Type of Engagement** & **Accuracy \({}^{\dagger}\)** & **Practicality \({}^{\star}\)** & **Application** \\ \hline \multirow{4}{*}{Data logging} & Questionnaire & Cognitive & L & Weak & Learner Assessment; \\  & Interview & Cognitive & L & Weak & User Experience [34]; \\  & Observer Scores & Behavioural & L & Weak & Human–Robot \\  & Mouse Clicks & Behavioural & M & Strong & Interaction [25] \\  & Web/App Access Duration & Behavioural & M & Strong & \\ \hline \multirow{4}{*}{Eye tracker} & Gaze & Behavioural & H & Medium & Driving [37]; \\  & Fixation & Behavioural & H & Weak & Social Interaction [123]; \\  & Saccade & Behavioural & H & Weak & ADHD Diagnosis [73]; \\  & Blink & Behavioural & H & Medium & Human–Robot \\  & ROI Events & Behavioural & H & Medium & Interaction [82] \\  & Head Pose & Behavioural & M & Strong & \\ \hline \multirow{4}{*}{Camera} & Facial Action Units & Behavioural and Emotional & H & Strong & \\  & Sentiment & Emotional & M & Strong & Driving [38]; \\  & Eye State & Behavioural & H & Strong & E-learning [57]; \\  & Posture & Behavioural & M & Strong & Social Interaction; \\  & Gesture & Behavioural & M & Strong & Human–Robot \\  & Head Pose & Behavioural & M & Medium & Interaction [14] \\  & Lip Motion & Behavioural & L & Strong & \\ \hline \multirow{4}{*}{Laser} & Proximity & Behavioural & L & Weak & \\  & Posture & Behavioural & M & Weak & Human–Robot \\  & Gesture & Behavioural & M & Weak & Interaction [91] \\  & Head Pose & Behavioural & M & Weak & \\ \hline \multirow{4}{*}{Physiological sensor} & EEG & Emotional and Cognitive & H & Weak & Driving [37]; \\  & EOG & Emotional and Cognitive & H & Weak & Education [112]; \\  & Facial EMG & Emotional & M & Weak & ADHD Diagnosis [126]; \\  & GSR & Emotional and Cognitive & M & Weak & Social Interactions [113]; \\  & Heart Rate & Emotional and Cognitive & M & Medium & Biofeedback for \\  & Blood Pressure & Emotional and Cognitive & L & Medium & Training [127] \\  & Respiration & Emotional and Cognitive & L & Medium & \\ \hline \multirow{4}{*}{Audio recorder} & Speech & Behavioural and Emotional & H & Strong & Social Interaction [21]; \\  & Non-speech Acoustic Cue & Behavioural and Emotional & M & Medium & Human–Robot \\  & Floor & Behavioural & M & Strong & Interaction [12] \\ \hline \hline \end{tabular} \({}^{\dagger}\) Accuracy grade (H)/Medium (M)/Low (L) is determined by self-reported accuracy performance in the literature [2, 3, 9, 14, 25, 29, 34, 37, 38, 56, 57, 58, 59, 70, 82, 105, 108, 110, 110]. Not all references directly deal with engagement estimation. Estimation results of human affects, fatigue, and human activity recognition are accepted as ascribing to sensor cues. In case difficulty of the evaluation/experiment/dataset varies cross-publication, we adjust the accuracy grade depending on a reference’s year of release. \(\star\) Practicality specifies general friendliness of deployment. We mainly consider five aspects: _form factor, ease to set up, intrusiveness, cost,_ and _user acceptability_. As sensor technologies keep evolving, here we take cues extracted from the most advanced model of each sensor to the best of our knowledge [2, 21, 25, 27, 30, 37, 69, 78, 91, 105, 108, 110]._  
\end{table}
Table 3: Sensor modalities useful for human engagement estimation.