In ohmic discharges in all tokamaks at low plasma densities the global energy confinement time, , increases almost linearly with the density (LOC, linear ohmic confinement). In tokamaks with sufficiently large dimensions, saturates at a critical density (ASDEX: ) and is nearly constant at higher densities (SOC, saturated ohmic confinement). In the same density region some experiments report a further confinement regime for deuterium discharges in which exceeds the saturated value and is further increased (IOC, improved ohmic confinement). There the global energy confinement time roughly behaves as in the LOC regime. For both the LOC and the SOC regimes an isotope effect, i.e. the dependence of on the ion mass, is reported as an additional aspect of the ohmic energy confinement. A statistical analysis is performed to identify the parameters which are responsible for the properties of the energy confinement in these discharges in ASDEX. In contrast to earlier reports on confinement time scalings in ASDEX OH, only discharges with a full experimental description of kinetic electron and ion parameters, i.e. profiles of densities, temperatures and , are used to evaluate the energy contents of both species. By means of statistics it is shown that the characteristics of are mainly caused by the behaviour of the electron energy flux and the ohmic input power. The ion energy flux, however, does not play a significant role. Furthermore, the IOC regime is explained as a continuation of the low-density LOC regime. Both the isotope effect and the density dependence of are caused by features of the electron energy transport.