Clinical trials use a number of techniques designed to reduce bias

The reviews on Amazon were found to be overwhelmingly positive

“Don’t believe online reviews of health products, they’re ‘skewed’,” the Mail Online reports.

A psychologist compared online reviews of three medical products with results from clinical trials, and found the reviews are skewed towards the positive.

The author of the study, Dr Micheál de Barra, wanted to look into whether people who have had good outcomes from treatments are more likely to go online and give positive reviews than people who have had average or poor outcomes. As such, the product reviews provided by online retailers may be distorted.

The author looked at – the US version of the site – and analysed two cholesterol-reducing products and one weight loss treatment.

In general, he found the extent of cholesterol reduction or weight loss reported by online reviewers was substantially greater than that demonstrated in randomised controlled trials, a more reliable source of evidence on effectiveness.

The research highlights an underlying issue with online reviews, whether they are for health products, films or books. Online reviews are arguably subject to a type of reporting bias in that they are written by people who take the time to write them.

This means it’s far more likely that these reviews are written by people who have very strong views, either positive or negative, about a product than people who would score it three out of five stars.

Useful impartial online resources for assessing the effectiveness of medical products and treatments include NHS Evidence, the TRIP Database and the Cochrane Library.