A study conducted on 6,500 people in Chicago, published online by the journal of the American Academy of Neurology, found no link between a person's educational level and their rate of decline to Alzheimer's and other forms of dementia.

"Some studies in the past suggested education helps people stave off the disease, it's more likely that the better schooled simply have farther to fall before the impact becomes noticeable", said Robert Wilson, a professor at Rush University in Chicago who led the study.

"It looks like things that are indicators of socioeconomic status don't affect your underlying risk once dementia is detected", Wilson said.

"Education provides you a little bit of a cushion, if you have a higher level, but it's no absolute protection."

Alzheimer's, a disease that impairs memory, language and other mental abilities, afflicts about 18 million people worldwide, a figure expected to double by 2025, according to the World Health Organization.

Chicago's neighbourhood was tracked down for over 14 years, testing memory, problem-solving and other skills at three-year intervals. The length of the study and frequency of testing made the research more accurate than past reviews, Wilson said.

The subjects, whose average age was 72, had educational levels ranging from eight years of schooling or less to 16 years or more. Other factors such as race and occupation also had no impact on the rate of decline, the authors said.

Alzheimer's mainly affects people older than age 60 and staying mentally and physically active has been shown to reduce the risk of developing dementia and its impact. The new findings don't change that advice, Wilson said.