Brenda's loyalty lays her private life secrets open

Brenda is a large woman whose desire to lose weight is thwarted by her appetite. She has long hair, bad skin and is shortsighted. Her parents are still alive, she lives for holidays and has a long-term boyfriend.

Such is the snapshot of a modern British woman as gleaned from the data collected by a single store loyalty card.

Research by Andrew Smith, senior lecturer in marketing at Nottingham University Business School, and Leigh Sparks, professor of retail studies at Stirling University, reveals the unrivalled power of retailers'

databases to illuminate the behaviour of the public.

As part of the project, the academics focused on one anonymised consumer - dubbed Brenda - whose purchasing habits stretched over two years with a branch of a major high-street retailer.

Many facets of Brenda's life were laid bare by data collected through her store loyalty card.

Large-sized tights revealed a large woman. Her food-buying patterns revealed faddish, yet healthy, eating habits, although usually consisting of big portions. She also bought a set of weighing scales in the two-year period.

The hair accessories she bought betrayed long hair, contact lens-cleaning products poor eyesight and blemish concealer bad skin. The purchase of fake tanning products heralded fortnight-long gaps in her record, revealing when she took holidays abroad.

Christmas cards - bought in October - introduced her parents and partner, for whom she also bought male grooming products.

There were also insights into Brenda's personality in the regularity and nature of her shopping behaviour.

Dr Smith accepted there was nothing wrong with the practice, but he was concerned at the ethical and privacy implications of vast quantities of sometimes personal information this involved.

The majority of British people regularly use loyalty cards, prompted to betray their shopping habits in return for a small financial reward.

"Loyalty cards are not about loyalty, they are about collecting consumer data for targeting communications, marketing and offers," he said. "We give a very high level of detail and insights into our lives, some of it very sensitive, to the retailers."

Purchasing patterns could potentially reveal the onset of addiction to prescription medicines, bouts of heavy drinking and sexual habits.

Stores do not routinely analyse their databases on an individual level and do not share them with the government, instead using them to better target marketing initiatives. But Dr Smith noted that beyond basic legal obligation retailers' responsibilities towards loyalty cardholders were not clear.

The study will be published in European Advances in Consumer Research.