# Handheld Optical System for Pectus Excavatum Assessment
## 2 Materials and Methods
### Usability Test Design  
The application's graphical interface was designed to be simple, clean, intuitive, and reliable. The result is a tool with fast access to the three main areas, which are the toolbar, the 3D navigator, and the patient-information form. The program was designed to be used independently by the medical staff and by the end user in a domestic setup; consequently, it was of crucial importance to assess the actual degree of acceptance and user-friendliness of the application by means of a usability test.  
The tasks on which to conduct the usability tests were identified as follows:  
Figure 5: The correction index 3D (CI3D).  
Figure 6: Simplified GUI for patient usage.  
Task 1: Chest acquisition. This involves the patient's positioning on the semirigid mattress and the acquisition, after positioning the camera approximately perpendicular to the patient's chest. Specifically, the doctor can open the video stream of the camera to help the correct positioning of the patient, after which they can perform the scan by using the proper button. The scan takes less than one second.  
Task 2: Correction index 3D (CI3D), area of the defect, and width computation. This task is carried out when the scan is ready and requires the use of the software to measure severity indices. To pursue this task, the user is required to manually select the area of the chest containing the defect. Once the area has been selected, the program automatically measures the three indices by intersecting the scan with a parallel sheaf of planes to detect the region of maximum depression. Subsequently, it measures the percentage of depression (CI3D), the distance of the two most protuberant points (width), and the area subtended between the line connecting these two points and the chest profile (area). Once the indices have been calculated (in approximately 3 seconds) the program shows the results to the user within a message box.  
The following metrics were chosen according to the ISO 9126 series of standards [29] to assess the usability of the program:  
Effectiveness, which is calculated by measuring the completion rate, and assigning a binary value of 1 if the test participant manages to complete a task, and a value of 0 otherwise.  
\[Effectiveness=\frac{number\ of\ tasks\ completed\ successfully}{total\ number\ of\ tasks\ undertaken} \tag{1}\]  
Efficiency, which is measured in terms of task time, i.e., the time spent by the users to achieve the goals.  
\[Time\ Based\ Efficiency=\frac{\sum_{j=1}^{R}\sum_{i=1}^{N}\frac{n_{ij}}{T_{ij}}}{NR} \tag{2}\]  
\[Overall\ Relative\ Efficiency=\frac{\sum_{j=1}^{R}\sum_{i=1}^{N}n_{ij}*t_{ij}}{ \sum_{j=1}^{R}\sum_{i=1}^{N}t_{ij}} \tag{3}\]  
where \(N\) is the total number of tasks, \(R\) is the number of users, \(n_{ij}\) is the result of task \(i\) by \(j\)th user, \(t_{ij}\) is the time spent by user \(j\) to complete task \(i\). Efficiency, which is the speed of work with the product, is typically compared with the expert efficiency, the highest theoretically possible speed of work.  
At the end of each session, users were asked to compile a system usability scale (SUS) questionnaire to measure their impression of the overall ease of use of the system. The SUS provided a quick and reliable tool for measuring the usability. It consisted of a 10-item questionnaire with 5 response options, ranging from \"strongly agree\" to \"strongly disagree.\" To interpret the results of the questionnaire, the participant's scores for each question were added together and then multiplied by 2.5 to convert the original scores of 0-40 to 0-100. Although the scores ranged from 0-100, they did not represent percentages, and should be considered only in terms of their percentile ranking: values below 51 were considered as strongly insufficient, 68 was considered to be the sufficiency threshold, and results above 80.3 were considered optimal.