## 4 Discussion
### Sources of Error and Precautionary Steps
Ultrasound and MRI techniques have the potential to somehow be included in this error because both are very time-consuming, taking between 592 s [41] and 780 s [42; 60] to complete the measurement. Like most of the techniques, both techniques are also very sensitive to the subjects' movement, which will later cause errors in the final residual limb volume reading. Researchers should take extra precautions to avoid evaluating the residual limb volume incorrectly. Precautionary steps need to be taken while experimenting, especially when it involves repeated session tests. Researchers should monitor the subjects' activity using activity monitoring devices such as those utilised by [PERSON] et al. [54], as different amounts of activity levels may directly affect the results. Activity monitoring devices may monitor the activity to give a similar amount of activity levels. For example, in between morning and afternoon sessions, the subjects are allowed to have their own activity without being monitored. For some subjects, they can just sit down outside of the lab for three hours, while on another day, they perform various activities. These situations may cause inaccurate results of the residual limb volume changes and directly give the wrong interpretation of the residual limb volume fluctuation. Other than that, the marker placement or position during the experiment is also important throughout all the experiment sessions. Different marker positions, especially for immersion indication in the water displacement technique, would cause different amounts of water to be displaced from the tank in every different measurement session. It is good to use an ink marker that cannot be rubbed off and displaced throughout the sessions. Some techniques such as water displacement, MRI scanning, and SXCT technology necessitate the subjects or residual limb not to move until the measurement session is complete. To minimise erroneous interpretations of the residual lower limb volume in the subjects who cannot stand still or who do not move during the measurement sessions, different procedures are preferable. Furthermore, all nine techniques except for SXCT and bioimpedance can measure the residual limb outside of the prosthetic socket. Precautionary steps need to be taken as the residual limb volume will affect the post-dofling volume change effect. Fast capture techniques such as bioimpedance, SXCT, and optical or laser scanning need to be used to investigate the post-dofling effect as opposed to the slow measuring techniques such as MRI, ultrasound, contact probes, and water displacement. In summary, all measurement techniques can lead to errors. There are four potential sources of error associated with the residual limb volume measurement techniques for the lower limb, which include (1) subject or residual limb movement, (2) the distortion of the residual limb's shape, (3) the post-doffing effect, and (4) the resolution and precision of the used device. Also, according to the authors' findings, there are three main causes of this sort of error, which are (1) the individual performing the measuring, (2) the subject being measured, and (3) the measuring device itself. Researchers can minimise potential errors while measuring the residual limb volume by (1) monitoring the subjects' activity during the measurement session, (2) using high-quality markers that cannot be easily rubbed off and avoiding different marker placements, and (3) choosing the right techniques according to the aim of the research and avoiding using measuring techniques that require a lengthy time to reduce subject movement. Correct precautionary steps may reduce errors while measuring and lead to a more accurate interpretation of the residual limb volume.