A random sample of non-institutionalised Danes aged 40+ years was interviewed face-to-face through computer-assisted personal interviews by Gallup Inc, Denmark. Questions were concurrently presented to the respondents orally and in writing on cards.
A sample of 3,548 individuals aged 40+ years was randomly drawn from a national database at Statistics Denmark. The sampling ensured geographic representativity. A total of 415 individuals could not be contacted because they were either non fluent in Danish (n = 62), suffered from sickness, senile dementia or reduced hearing (n = 154) or because the address identified was non-inhabited, non-existing or used for industrial purposes (n = 199)). The net sample consisted of 3,133 individuals. After three attempts, 731 potential respondents could still not be contacted, and 883 refused participation. All together 1,519 (49%) respondents completed an interview.
Information on age, gender, marital status, education and household income was collected. The two latter variables were included, because it is conceivable that understanding of risk information is higher in population groups with higher income or education. Respondents were also asked whether they were diagnosed with hypercholesterolaemia or had experienced a heart attack.
Additionally, the respondents were asked to consider a hypothetical intervention (see below). Similar questions have been used in previous studies conducted by Odense Risk Group, wherefore no pilot testing was performed. In order to test whether baseline risk had an effect on the acceptance of this intervention presented in terms of RRR, respondents were allocated to alternative versions of a case scenario in which baseline numeric risk information was either present or not. The following wording was used:
• Version 1:"Imagine that your GP tells you that you have a slightly increased risk of suffering a heart attack. On average, 10 out of 1000 patients like you will die of a heart attack within 3 years."
• Version 2:"Imagine that your GP tells you that you have a slightly increased risk of suffering a heart attack."
Subsequently, the following information was given to all respondents:
Your GP presents you with a medication, which should be taken once a day. The medication has mild and harmless side effects. The treatment requires that you visit your GP twice a year for a check-up. The annual cost of your medication is approximately 500 DKK (~£45), which you will have to pay yourself.
Your GP tells you that the use of the medication for 3 years will reduce your risk of heart attack by X%."
The impact on choice of the magnitude of RRR was tested by randomly allocating respondents to varying X = 10, 20, 30, 40, 50, 60. We chose these levels because most medical interventions attain effectiveness within this range. RRR was presented as percentages because this is the way they are usually presented in the medical literature even though relative frequencies may be easier to understand [3]. By random, half of the respondents in each RRR group were presented with the baseline risk of heart attack, while the others were not. The randomisation was done by a computer at the start of the interview.
Subsequently, the respondents were asked whether they would choose to take the medication, and also asked to answer a question regarding their perceived difficulty of understanding the RRR information. The following preset answer categories were presented:
"Was it difficult to understand the size of the treatment effect?"
• Not difficult to understand
• A little difficult to understand
• Very difficult to understand
• Impossible to understand
In the subsequent analyses, the responses were recoded into a dichotomous variable, where one category represented those who had no difficulties understanding the case, and the other category represented the remaining respondents.
Variation in consent with increasing RRR was tested with bivariate trend analysis. Additionally, logistic regression was performed to explore determinants of consent including presentation of baseline risk.
With 125 respondents in each RRR-group, we had a power of 99% to detect a trend describing an increase from 40% to 75% acceptance of therapy with increasing effectiveness. A similar effect has been observed in a study of prolongation of life as measure of benefit [12].