Table 2 Summary of identified themes

1. What design features shape smokers’ and drinkers’ choice of apps?

The immediate look and feel of the app

First impressions of the app’s aesthetic appeal (e.g. colour scheme, minimalist design) and usability (e.g. easy to understand, not too text-heavy).

Social proof

The app’s perceived quality, largely determined by ‘social proof’ (i.e. other users’ ratings, recognition of credible brands/institutions).

Realistic and relevant titles

Titles that appeared realistic and relevant to the target behaviour (e.g. “quit smoking”, “reduce your drinking”).

2. What design features are judged to be important for engagement?

Features that enhance motivation

Features that enhanced participants’ motivation to stay smoke-free/reduce their drinking (e.g. monitoring and feedback, goal setting, rewards).

Features that enhance autonomy

Features that enhanced participants’ autonomy (e.g. user-controlled reminders, flexible quitting/reduction plans).

Features that enhance personal relevance

Features that engendered a sense of personal relevance (e.g. tailoring of content, a non-judgmental communication style, gain-framed messages).

Features that enhance credibility

Features that engendered a sense of credibility and trust (e.g. a clear privacy policy, information perceived to be accurate).

Consistency with online and offline social preferences

Consistency with participants’ attitudes towards sharing progress on social media or joining an online support community (i.e. online preferences) and their attitudes towards using the app to log cigarettes/units of alcohol or distract from cravings in social settings (i.e. offline preferences).