A new study by Stanford University scholars Jon Krosnick and Neil Malhotra shows that most Americans recognise that smoking can lead to life-threatening diseases, but they don't understand how much that risk increases.

The cause, the researchers assumed, lies in the misperception of the risk.

Published in the scientific journal PLOS ONE, the study analysed data from interviews with more than 13,000 adults in the United States, comprising smokers and non-smokers, about the prospects of developing lung cancer.

In some previous studies, respondents were asked how likely they thought smokers and non-smokers would develop lung cancer.

If a respondent thought smokers had a 30 per cent chance of developing lung cancer and non- smokers a 10 per cent chance, that person thinks that smokers are 20 percentage points more likely to develop cancer.

In doing the new study, the researchers - Mr Krosnick is Stanford's professor of communication and political science and Mr Malhotra is professor of political economy in the Graduate School of Business - realised that some people might not be thinking about risk that way.

Instead, they said, people might perceive risk as the ratio of the two numbers, like answering the question, "How many times more likely to get lung cancer is a smoker than a non-smoker?"

This ratio is called "relative risk". In the 30 per cent vs 10 per cent example, a person thinks of the smoker as three times more likely to develop lung cancer.

This seemingly small change in analytic approach has big consequences - most Americans overestimate the difference between the two rates of lung cancer, namely the risk of smoking, but the vast majority underestimate the relative risk. In essence, they underestimate how much more likely it is for a smoker to develop lung cancer compared to a non-smoker.

The researchers did find that people who perceived more relative risk were less likely to start and more likely to quit smoking, and those who perceived more of a difference between the two cancer rates were no more or less likely to start or stop smoking.

Therefore, they suggested that if people think about such dangers in terms of a ratio, then, perhaps, rates of smoking would be reduced if Americans are properly informed about such ratios.

Cigarette-packet labels in the United States include warnings from the Surgeon-General that smoking can cause specific health problems, without giving numeric figures to quantify the impact.

In Australia, cigarette labels include quantitative data such as, "Tobacco smoking causes more than four times the number of deaths caused by car accidents".

Prof Krosnick, the lead author, was quoted as saying: "Telling people how many times a person's chances of getting lung cancer increase due to smoking may help the public make more informed choices."

XINHUA