The construct of the POSAS (i.e. what it aims to measure) is scar quality. Scar quality is formed by the scar characteristics included as items in the POSAS. Therefore, if the aim is to evaluate scar quality, the sum score must always be used.
The Patient Scale and the Observer Scale are two separate instruments that measure quality from different perspectives, and, therefore, are complementary to each other. The sum score of both instruments provides no additional meaning.