Many doctors are not convinced of a link between iron levels and heart disease in men.

CNN: Health: Making Heartfelt Gestures

As they believe that there is a link between deprivation and lower levels of literacy, they called on the government to implement a pupil premium to address this.

BBC: Debate on literacy

They carried out detailed statistical analysis, in order to assess whether there was a link between cases of cancer and above average levels of radiation.

BBC: Radon cancer fears dismissed

In fact, at slightly lower levels of income, the link is as strong as ever.

ECONOMIST: Banyan

Many epidemiological studies link low vitamin D levels to a high risk of getting or dying from various cancers down the road, especially colon cancer.

FORBES: Magazine Article

The Home Office has been seeking to tighten up routes of entry into the UK and break the link between temporary migration and permanent settlement as part of efforts to substantially reduce levels of net migration.

BBC: Graduates from a British university

In education, the evidence is less clear: international studies of teenagers' prowess in mathematics and science show no clear link to spending levels.

ECONOMIST: Health and education

The problem with many of the studies that supposedly establish a link between neonicotinoids and bee deaths is that they fail to simulate the exposure levels bees are likely to encounter in the wild.

FORBES: Bee-ing Smart: Regulators Must Distinguish Activists' Bad Dreams From Good Evidence