Systematic review- we will review the evidence that already exists in medical journals and research articles to understand how we can better conduct our research and understand what is already known about this issue.

Exploratory study- we will see if DIALOG+ is acceptable by asking people with depression their opinions on the app so we can adapt it to their needs. This will involve getting people with chronic depression to use the app-based intervention and gaining feedback from them. We will also speak with mental health professionals to find out their views.

Feasibility trial- we will run a small study with 9 staff members and 45 service users with chronic depression to gain further experience with DIALOG+ and check whether a larger study would be possible and feasible.

Randomised Controlled Trial (RCT)- RCTs are the ‘gold standard’ in medical research for understanding if a treatment or intervention is effective. We will therefore conduct an RCT in around 9 sites across the UK to test whether DIALOG+ really helps patients with chronic depression. This will include 112 care coordinators and 448 patients, half of whom will use DIALOG+. The other half will not use DIALOG+, and will act as a comparison group. We will ask all participants to fill out questionnaires at the start of the study and after 6 and 12 months. We will then compare the 2 groups using these questionnaires to see if patients receiving DIALOG+ show improvements over time (in comparison to the control group). This study will also help us to see if using DIALOG+ on chronic depression will save the NHS money, compared to standard treatments that are used currently.

Process evaluation- we will speak to the people who participated in the RCT, and clinical staff who helped us run it, to better understand our findings. We will look at things such as why some people declined to take part or how easy it was to deliver the intervention within NHS services to get a better sense of what the findings are telling us.

Staff training module- if the approach is found to be successful, we will develop a training programme for different health professionals to see how training for DIALOG+ may need to differ.

Dissemination- throughout the project we will make sure our findings are presented and published. We will write articles in research journals and present at academic conferences, so the wider scientific community know what we are doing and what are finding out. We plan to work closely with NHS Trusts to ensure that findings are presented in newsletters and 'research days'. We will also present to community service user groups, and engage with the general public. At the end of the programme, there will be a targeted period of dissemination to ensure key stakeholders (e.g. mental health service commissioners, clinicians, service users, carers and academics) are aware of the results of TACK.