# A Three-Class Classification of Cognitive Workload Based on EEG Spectral Data
## 2 Materials and Methods  
The experiment was conducted in quiet laboratory conditions by using a dedicated 21-channel Mitsar EEG 201 amplifier. The signal was gathered from 19 electrodes, and a computer mouse was used in order to provide the answers. A special cap and EEG cup gel were applied in order to ensure good conductivity in the experiment. The electrodes were placed according to the 10-20 system. The monopolar average montage was applied, and the ground electrode was placed in the center of the frontal lobe of the person examined. Twelve male participants took part in the experiment, but the signal from one of them contained a high level of noise and, due to that fact, only the signals from 11 subjects were used in the further analysis. The mean age of the people examined was 22 years. The subjects were asked to minimize movements and blinking in order to obtain less noisy fragments in the signal. The samples were recorded with the frequency of 500 Hz. The experiment lasted about 20 min, and all participants had to carry out the same of tasks. The procedure of the experiment was divided into six intervals of advancement. The person was prepared for the examination, and the calibration was performed before the first part. Each interval except for the last one (interval 6) consisted of 17 arithmetical tasks, and the consequent intervals were separated with breaks. The last interval included 16 arithmetical tasks, as the last stimulus was not fully recorded. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Paper**} & **Workload Class** & **Subject** & **Classifier** & **Metric** & **Metric** & **Approach** \\  & **Number** & **Number** & **Number** & & & \\ \hline
[40] & 2 & 31 & LDA, LIBLINEAR, kNN, LIBSVM & accuracy & 0.95 & sd \\ \hline
[35] & 2 & 6 & SVM & accuracy & 0.93 & sd \\ \hline
[41] & 3 & 96 & SVM & mistrust rate & 0.29 & sd \\ \hline
[36] & 2 & 28 & SVM, k-Means, kNN & accuracy &
\begin{tabular}{c} 0.92, 0.86; \\ 0.58 \\ \end{tabular} & sd \\ \hline
[53] & 3 & 26 &
\begin{tabular}{c} Decision tree, \\ discriminant analysis, \\ logistic regression, SVM, \\ kNN, ensemble classifier \\ \end{tabular} & accuracy & 0.84–0.89 & sd \\ \hline
[54] & 2 & 30 & SVM & accuracy & 0.58–0.89 & si \\ \hline
[48] & 5 & 10 & SVM & accuracy & 0.91 & sd \\ \hline
[42] & 5 & 12 & Random forest & accuracy & 0.87 & sd \\ \hline
[50] & 2 & 18 & ELM & accuracy & 0.93 & sd \\ \cline{3-6}  & 2 & 18 &
\begin{tabular}{c} kNN, decision tree \\ ANOVA, CFT \\ \end{tabular} & accuracy & 0.92 & sd \\ \hline
[43] & 2 & 10 & SVM & accuracy & 0.97 & sd \\ \hline
[44] & & & kNN & accuracy & 0.8 & sd \\ \hline
[55] & 2 & 12 & MLP & accuracy & 0.99 & si \\ \cline{3-6}  & 4 & & & 0.88 & \\ \hline
[51] & 5 & 16 & LDA, kNN & accuracy & 0.96 & sd \\ \hline
[33] & 3 & 8 &
\begin{tabular}{c} hierarchical Bayes model, \\ neural net, naive Bayes \\ \end{tabular} & accuracy & 0.8 & si \\ \hline
[56] & 4 & 9 & SVM, LDA & accuracy & 0.56 & sd \\ \hline \hline \end{tabular}
\end{table} 
Table 1: _Cont._In total, each conducted experiment contained 6 intervals and 5 breaks. Each task was based on displaying a mathematical operation (adding, subtracting, multiplying integers) and the result of this operation. The person examined had to make the decision if the result was correct and then click on the button of the mouse, or do nothing if the answer was incorrect. The appearance of the stimulus was marked by a marker in the record, and the moment when the user clicked was marked as well. Figure 1 below presents the overall description of the conducted experiment with the numbers of intervals, resting breaks, and stimuli. Figure 2 shows the procedure of a single stimulus: The stimuli are separated with 1 s long breaks, after a break a task was demonstrated, shortly after the task the answer was provided, after that the participant was given approximately 3 s for approving/disapproving the answer by clicking/not clicking a mouse button, after that the next cycle was started with 1 s long break [49; 52; 57].