## 4 Discussion  
In this section, the experimental results reported previously are analyzed and discussed to evaluate the performance of the developed wearable device. Then, a performance comparison with similar systems presented in the literature is presented. Considering the seated user, the correlation coefficient was equal to 0.958, whereas it was 0.956 when standing and 0.923 when the user walks (Figure 10). The high value of the Pearson coefficient indicates that the measurements of the chest band are strongly correlated with the corresponding measurements obtained by counting breaths, which are used as a reference. However, considering the measurements for walking users (r = 0.923), a slight degradation of the sensor accuracy was observed compared to stationary users (r = 0.958--seated user, and r = 0.956--standing user). The discrepancy between measurements is caused by false peaks induced by the body movements, which were not removed by analyzing the inertial data acquired by the onboard accelerometer. However, the device performance can be enhanced by optimizing the acceleration threshold involved in the detection algorithm and the cut-off frequency of the digital lowpass frequency; in contrast, no difference in performance was observed when a seated or standing user wore the chest band. Analyzing the Bland-Altman plots of the measurements obtained in the three operative conditions, different information related to the data concordance between the chest band and reference measurements can be inferred. Specifically, the test results indicated a mean difference (MD) of 0.8 Breaths Per Minute (BrPM) and an LoA (Limit of Agreement) equal to +3.47 and -1.8 BrPM when the user was seated (Figure 11a). On the contrary, the chest band resulted in an MD of 0.4 BrPM and LoA equal to +3.40 and -2.61 BrPM when the user was standing (Figure 11b). Finally, an MD of 0.2 BrPM and LoA equal to +3.77 and -3.37 BrPM was achieved when the user walked (Figure 11c). The results show that the piezoresistive chest band shows a lower bias component (MD) when the user is walking, compared to when they are steady. However, a higher LoA indicates a higher possible error in RR measurement. In addition, for all test conditions, the measurement points appear uniformly distributed as the mean value varies, indicating that the error between the two methods is uniform as a function of the measurement value. Finally, the obtained results demonstrate that the developed respiratory sensor is reliable, and provided a good estimation of the respiration rate in daily life conditions. Observing the signal acquired by the chest band, reported in Figure 9, it is evident that this covers a limited number of quantization steps, thus limiting the resolution of the acquired signal. As a future development, an increase in the amplifier stage's gain would  
Figure 11: Bland–Altman graphs of the measurements obtained by the developed chest band and those obtained by breath counting when the user is: seated (**a**), standing (**b**), and walking (**c**). further improve the performance of the chest band in detecting the breathing movements (inhalation and exhalation), and thus the corresponding parameters. Next, preliminary tests were performed to establish the performance of the developed device in measuring the other breathing parameters (TI, TE, IER, and V). As a reference, the measurements provided by the portable spirometer (Contec SP10) were acquired concurrently with those provided by the chest band. Specifically, the TI and TE values related to a single breath were considered, in addition to the corresponding flow rate. Table 2 summarizes the results of the measurements obtained by the chest band and the spirometer (used as a reference), and the corresponding errors. The obtained results indicate an optimal agreement between the measurements of the chest band and the reference values, with an error of less than 8% on all acquired parameters, thus confirming the correct operation of the developed device.