From first principles of plasma physics the local thermal diffusivity can be expressed in terms of the Bohm diffusivity cB and a function F of dimensionless parameters. Tests of transport models, theoretical, empirical or heuristic, expressed in terms of such parameters often produce ambiguous answers which arise from collinearities in the experimental data. In experiments on dimensionally similar discharges the scaling of confinement with one single dimensionless parameter can be examined in a more un-ambiguous fashion. Experiments on JET have been carried out in which only the normalised Larmor radius p. is varied. This is achieved by varying the plasma current, the toroidal field, the density, the ion cyclotron frequency and power level in a predetermined pattern such that other dimensionless parameters like normalised collisionality n*, b and safety factor qy are kept constant. The experiments on JET demonstrate that L-mode confinement both globally and locally scales with r* according to the long wavelength or Bohm scaling. The implications for theory and future experiments are outlined.