Bowel cancer is the second biggest cancer killer in the UK, accounting for over 16,000 deaths per year. Screening can reduce deaths from bowel cancer if the people invited participate. The challenge is that high uptake of bowel screening is hard to achieve, and remains persistently below 65%.

The faecal immunochemical test (FIT) is the most widely used bowel screening test worldwide.

In the UK, FIT kits are mailed to people's homes without guidance on when the kit should be returned and only brief instruction on how to use it. Some people have said that even though they intend to complete and return the kit, they often forget or put off doing it.

Two approaches are proposed to addressing this issue: i) providing a suggested deadline for FIT return, because it is known from breast and cervical cancer screening that giving people an appointment time increases uptake compared to an open invitation, and ii) planning sheets, that have been found to help people act on their intentions in other health contexts.

This trial aims to evaluate the impact of providing a suggested deadline and a planning sheet on the return of FIT bowel screening kits.

The trial is integrated within the Scottish Bowel Screening Programme. The investigators will randomly allocate 40,000 consecutive people that are due to be sent a FIT kit to one of eight groups:

(i) control group (no deadline, no planning sheet), (ii) intervention group (1-week deadline, no planning sheet), (iii) intervention group (2-week deadline, no planning sheet), (iv) intervention group (4-week deadline, no planning sheet), (v) intervention group (no deadline, with planning sheet), (vi) intervention group (1-week deadline, with planning sheet), (vii) intervention group (2-week deadline, with planning sheet), (viii) intervention group (4-week deadline, with planning sheet).

It will then be examined if having a suggested deadline and a planning sheet affects how many people send back their completed FIT kit. It will also be examined if the deadline length makes a difference and whether having both a deadline and a planning sheet affects the number of people returning their kit.

Finally, the cognitive and behavioural mechanisms underlying any intervention effects will be assessed and the acceptability of the interventions explored, using questionnaires and in-depth interviews.

Inclusion Criteria:

• 50-74 years
• Registered with a Community Health Index number in Scotland
• More than 2 years since last bowel screening invitation

Exclusion Criteria:

• Has self-excluded from Scottish Bowel Screening Programme
• Not sent a screening kit by the Scottish Bowel Screening Programme

Outcomes 1-4: Details of the dataset and work arising from it will be available from the Robertson Centre for Biostatistics Analytical Platform at the University of Glasgow. The dataset will be archived on live servers within the facility. Use of anonymised data within research is given for the specified project purpose, future use is permitted but would require permission from LPAC and the Caldicott Guardian.

Outcomes 5-6: Quantitative data (SPSS files with metadata) and qualitative data (stored as MSword transcripts with metadata) will be deposited in Enlighten: Research Data, the University of Glasgow's institutional data repository. Data in the repository will be stored in accordance with funder and University data policies. Files will be given a Digital Object Identifier (DOI) and the associated metadata will be listed in the University of Glasgow Research Data Registry and the DataCite metadata store.

Outcomes 1-4: 5 years post completion and data then moved to off-line storage.

Outcomes 5-6: 10 years from date of deposition in the first instance, with extensions applied to datasets which are subsequently accessed.

Outcomes 1-4: There is no requirement for a data sharing agreement with the research team and no restrictions on data sharing. Any research group can approach and request permission to use datasets.

Outcomes 5-6: Data made public at the time of publication.