Author	A. Ghanouni
Author	C. von Wagner
Author	J. Wardle
Author	S. Halligan
Abstract	Colorectal cancer (CRC) is an important source of disease burden in the United Kingdom and developed world. It is often preventable through certain forms of screening of asymptomatic individuals, allowing for the timely detection and removal of pre-cancerous polyps (adenomas). It can also reduce mortality (in individuals who already have CRC) by detecting it at an earlier, more treatable stage. Computed Tomographic colonography (CTC) is a comparatively new test that is capable of detecting adenomas before they transition into cancer, meaning that it can identify people for whom polypectomy is warranted. Like the "gold standard" test of colonoscopy, it can detect most adenomas. It may also be perceived more positively than colonoscopy by screening invitees because the exam is less invasive, potentially increasing uptake and improving population health outcomes. Another possible advantage of CTC over colonoscopy is that it is possible to replace the burdensome full-laxative bowel preparation with reduced- or even non-laxative alternatives. However, these are likely to be associated with a reduction in sensitivity and specificity, resulting in more false negatives and false positives, and this may detract from its overall public acceptability. The main aims of this PhD were to investigate the public's views about the optimum method of carrying out CTC, taking into account the trade-offs involved in bowel preparation options, and test whether a particular form of CTC had the potential to increase screening uptake compared with other preventative CRC screening tests (colonoscopy and flexible sigmoidoscopy). Chapter 1 consists of the background to CRC, screening, and the relevant tests. Chapter 2 introduces the issues of screening test preferences and uptake. Chapter 3 (Study 1) reports on a qualitative discussion group study that served as an initial exploration of public perceptions, values and preferences regarding CTC and colonoscopy. Chapter 4 (Study 2) reports on a qualitative interview study that compared patients' experiences with CTC following non- or full-laxative preparation in order to inform a further qualitative study in Chapter 5 (Study 3), which explored public perceptions and preferences for different bowel preparations for CTC, specifically taking into account the trade-offs. A more systematic assessment was planned to quantify the extent to which preparation tolerability, sensitivity and specificity were valued and Chapter 6 (Study 4) consisted of a review of previous studies that used the selected approach (conjoint analysis) with the aim of identifying strengths and weaknesses in the existing literature. These findings were drawn on when designing Study 5 in Chapter 7, which consists of a particular form of conjoint analysis (a discrete choice experiment) to quantify public values of the main attributes of interest for CTC. The final study in Chapter 8 (Study 6) randomised participants to receive a hypothetical screening invitation for one of several preventative tests in order to measure how uptake of different forms of CTC might compare with colonoscopy and flexible sigmoidoscopy. Chapter 9 refers to findings from psychology that aims to broaden the perspective of how studies assess and interpret stated preferences for and perceptions regarding screening tests. Findings are then synthesised in Chapter 10, taking into account this broader literature. Participants consistently discriminated between CTC and colonoscopy across studies and appraised the former less negatively in terms of experiential characteristics. Participants were also consistent in anticipating the experience of non- and reduced-laxative preparation less negatively than full-laxative preparation. Results were more mixed in terms of appraisals of sensitivity and specificity in the context of CTC bowel preparation. The qualified interpretation presented here is that sensitivity is an influential attribute but specificity is not, when both are defined within a range of values considered plausible. Hence, although sensitivity and the anticipated experience of preparation were both important attributes, perceived gains arising from reducing preparation burden were offset by perceived costs from the corresponding reduction in diagnostic performance of CTC, leading to no clear overall preference for a particular preparation method. Furthermore, there was no evidence that screening uptake would be higher for different forms of CTC, or other preventive screening tests. The most robust method of confirming these findings would be direct comparisons of the tests in trials assessing actual screening behaviour.