Simple, to see what the brain cancer rate was when people were NOT exposed to ionizing radiation from cell phones held up against the head. Since there were none in 1982 to 1992 or so, it is a good control period to examine the rate of brain cancer rates against for similar periods later where people DID hold ionizing devices up against their heads. If the rates of brain cancer development are the same between a period when they did not hold ionizing radiation emitting devices against their heads and when they did, then it is most likely the ionizing radiation devices are not causing any increase in brain cancer.
What ionizing radiation is being emitted by cellphones?