There are several reasons for the discrepancies between other studies:

• Information about cell phone use, including the frequency of use and the duration of calls, has largely been assessed through questionnaires. The completeness and accuracy of the data collected during such interviews is dependent on the memory of the responding individuals. In case-control studies, individuals with brain tumors may remember cell phone use differently from healthy individuals, which can result in a problem known as recall bias.
  
  
• Digital cell phones have been in common use for less than a decade in the United States, and cellular technology continues to change. What was called 2G, or second-generation technology, was introduced in the United States in the 1990s. It was not until 3G, or third-generation technology, was introduced in 2001 that cell phone use became widely accepted in this country. Although older studies evaluated RF energy exposure from analog telephones, most cell phones today use digital technology, which operates at a different frequency and a lower power level than analog phones.
  
  
• The interval between exposure to a carcinogen and the clinical onset of a tumor may be many years or decades. Scientists have been unable to monitor large numbers of cell phone users for the length of time it might take for brain tumors to develop.
  
  
• Epidemiologic studies of cell phone use and brain cancer risk lack verifiable data about cumulative RF energy exposure over time (the total amount of RF energy individuals have encountered). These studies are also vulnerable to errors in the reporting of RF exposure by study participants. In addition, study participation rates are frequently different between those with cancer and those without cancer in brain tumor studies, a problem known as participation bias. Some studies have indicated greater participation by individuals diagnosed with brain tumors compared with control subjects, and participation rates may be related to cell phone use.
  
  
• The use of "hands-free" wireless technology is increasing and may alter cell phone RF energy exposure.

With the publication of the Interphone study, research has fairly consistently demonstrated that there is not a link between cell phone use and cancer, but scientists caution that further surveillance, especially of heavy users and children and adolescents, is needed before definite conclusions can be drawn.