Recall bias happened because the study was retrospective rather than prospective.

ECONOMIST: The Interphone study: Mobile madness | The

Second, in what's called "recall bias, " since people know the context in which they're being surveyed, "those with brain cancer tend to over-report historical exposure, " says Savitz.

FORBES

The most serious flaw in the study, however, is generally reckoned to be recall bias, which would tend to work in the opposite direction, by suggesting problems with phones that do not, in fact, exist.

ECONOMIST: The Interphone study: Mobile madness | The

The strengths of the study include the fact that it made use of company records to identify cell phone users, thus avoiding the bias involved in asking subjects to recall their past use.

FORBES: Do Cell Phones Cause Brain Cancer? The Diehards Cling Desperately To Opinion