**3. NUX-based approach for deriving sustainability-quality requirements**

In this section, we present the process needed for deriving sustainability-quality requirements from NUX results. As shown in **Figure 1**, the process consists of three stages: (i) UX assessment for understanding user needs, (ii) translating user needs into NFRs, and (iii) deriving sustainability-quality requirements from identified features and NFRs. The first two stages correspond to the NFR discovery approach proposed in [21], which uses the PSD model as a means to identify non-functional requirements (NFR) for a persuasive software system.

#### **3.1 UX assessment for understanding user needs**

UX assessment is supported by a wide range of research methods available, ranging from attitudinal evaluations (e.g. UX questionnaire, think-aloud) to behavioral evaluations (e.g. eye-tracking, activity trackers, emotions measurement). In this phase, in contrast to Sonnleitner et al. [22], we focus on negative User Experience (NUX) that is caused by the lack of fulfillment of needs during the interaction with a software product (a RSI software in our case). The effect of NUX impacts on the user attitudes ("what people say") and user behaviors ("what people do"). The outcome of this stage is the user feedback, information used for understanding user needs interacting with any persuasive software application.

**Figure 1.** *Deriving sustainability quality requirements from NUX results.*
