BACKGROUND: Web-based decision aids are known to have an effect on knowledge, 
attitude, and behavior; important components of informed decision making. We 
know what decision aids achieve in randomized controlled trials (RCTs), but we 
still know very little about how they are used and how this relates to the 
informed decision making outcome measures.
OBJECTIVE: To examine men's use of an online decision aid for prostate cancer 
screening using website transaction log files (web-logs), and to examine 
associations between usage and components of informed decision making.
METHODS: We conducted an observational web-log analysis of users of an online 
decision aid, Prosdex. Men between 50 and 75 years of age were recruited for an 
associated RCT from 26 general practices across South Wales, United Kingdom. Men 
allocated to one arm of the RCT were included in the current study. Time and 
usage data were derived from website log files. Components of informed decision 
making were measured by an online questionnaire.
RESULTS: Available for analysis were 82 web-logs. Overall, there was large 
variation in the use of Prosdex. The mean total time spent on the site was 20 
minutes. The mean number of pages accessed was 32 (SD 21) out of a possible 60 
pages. Significant associations were found between increased usage and increased 
knowledge (Spearman rank correlation [rho] = 0.69, P < .01), between increased 
usage and less favorable attitude towards PSA testing (rho = -0.52, P < .01), 
and between increased usage and reduced intention to undergo PSA testing (rho = 
-0.44, P < .01). A bimodal distribution identified two types of user: low access 
and high access users.
CONCLUSIONS: Increased usage of Prosdex leads to more informed decision making, 
the key aim of the UK Prostate Cancer Risk Management Programme. However, 
developers realistically have roughly 20 minutes to provide useful information 
that will support informed decision making when the patient uses a web-based 
interface. Future decision aids need to be developed with this limitation in 
mind. We recommend that web-log analysis should be an integral part of online 
decision aid development and analysis.
TRIAL REGISTRATION: ISRCTN48473735; 
http://www.controlled-trials.com/ISRCTN48473735 (Archived by WebCite at 
http://www.webcitation.org/5pqeF89tS).