## 2 Methods
### Dependent Variables  
According to the purpose of this study, the parameters of the physical characteristics and abilities of the ATCOs were considered as the dependent variables. The duties of ATCOs include staying focused, high visual and auditory perception, assessing air traffic conditions, making decisions, and issuing instructions. The physical states of ATCOs have crucial influences on these tasks. It has been shown that ATCOs need to pay high mental workloads to complete the tasks of perception, attention, information processing, and decision-making [57; 58; 59]. Therefore, the parameters in this study were selected according to these aspects. The parameters, equipment, and measurement methods are shown in Table 1. \begin{table}
\begin{tabular}{p{56.9 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt}} \hline \hline
**Characteristics** & **Equipment** & **Measurement Methods** & **Parameters (Dependent Variables)** \\ \hline \multirow{3}{*}{Physical state} & Two medical electronic blood pressure meters & The blood pressure and heart rate of the ATCO was measured in upper arm. & Heart rate (HR), systolic blood pressure (SBP), and diastolic blood pressure (DBP) \\ \hline \multirow{6}{*}{Attention} & & A regular triangle (sides of 200 mm and a width of 20 mm) was presented on the panel of the detector. The triangle lit up and rotated with uniform motion at 30 revolutions per minute. The ATCOs were required to track and aim the triangle continuously with an electronic probe. Each test lasted one minute, and the ATCOs had to maintain a high level of attention on the triangle. The successful tracking time (STT) and the number of deviations from the objective (NDFO) were recorded to indicate attention ability. \\ \cline{2-3}  & & The display screen of the detector showed a matrix with 16 \(\times\) 16 red points on a 120 \(\times\) 120 mm background. Five to sixteen red points were selected randomly and lit up for 0.5 s; the subjects had to remember the highlighted points and input their number. The test was repeated 36 times. The range of attention (ROA) was defined as the highest number that a subject input correctly with an accuracy rate of more than 50\%. \\ \hline \multirow{3}{*}{Reaction} & & Each ATCO was required to press a button after seeing a light to determine the reaction time to light (RTL) and after hearing a sound to determine the reaction time to sound (RTS). Each test was repeated 10 times, and the average reaction time was recorded. \\ \cline{1-1} \cline{2-3}  & & A 0.4 m long display screen was used on a detector. A red point moved 0.2 m with a uniform velocity of 0.1 m/s; subsequently, the red point was hidden and continued moving for the remaining 0.2 m at the same speed. The subject was required to perceive the speed of the red point and press a button when he anticipated that the point had reached the end. The time deviation between the actual time and the anticipated time was recorded. If the subject’s anticipated time was earlier than the actual time, the time deviation was positive; otherwise, it was negative. \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Equipment and measurement methods used to obtain the parameters. In Table 1, the parameters of HR, SBP, DBP, and DHR were used to represent physical characteristics, which indicate the basic physical state during work, and the other parameters explain the abilities of ATCOs. All the parameters indicating abilities were used to evaluate the ability levels of the ATCOs during the training, selection, and testing process in China. We also validated the relation between them and the states in the ATCOs through a subjective questionnaire [7]. The STT, NDFO, and ROA represented the characteristics of attention. Reaction ability was represented by the RTS and RTL. The TDSA is a measure of the accuracy of perceiving movement and is closely related to control timeliness. The LSPWB indicates the number of tests the ATCO was required to detect the correct spatial configuration.