This paper presents a design science evaluation of a vital sign chart for an Aggregate-Weighted Track-and-Trigger System (AWTTS) developed by the Irish Health Service Executive (HSE). This system is known as the National Early Warning Score (NEWS), with the chart incorporated within NEWS known as the Early Warning Score (EWS) Chart. Presentations of patient vital sign data on charts provides an opportunity for healthcare professionals (HCPs) to process vital sign data and identify trends of patient deterioration. Due to the reality of working in a complex work environment processing this data can be difficult for HCPs. This research evaluates how useful experienced nurses perceive the EWS chart to be in the context of supporting them in processing vital sign data presented on the NEWS chart. This paper represents a reflective data-gathering phase for the development of a design framework for improving how vital sign data is visualised on electronic AWTTS charts. This paper also utilises the approach for Reflective Design Knowledge Synthesis (RDKS) to synthesise design knowledge inherent to the NEWS chart. The empirical findings of this study involving interviews of 8 nurses with an average of 21 years' experience demonstrate that the NEWS chart is significantly more useful than legacy vital sign charts that would have been previously used. Furthermore, the evaluation establishes which design features are perceived to be most useful for nurses when processing vital signs displayed on the NEWS chart.