This paper analyses the properties of a critical gradient transport model based on a few assumptions: electrostatic gyroBohm scaling law, existence of an instability threshold, and finite background transport below the threshold. This model is characterised by only 3 scalar parameters. A quantitative criterion of stiffness is proposed, which provides a mean for a quantitative assessment and inter-machine comparison. It is also shown that this transport model is compatible with a two term scaling law of global confinement, as proposed recently by the ITPA-CDBM group (International Tokamak Physics Activity - Confinement Data Base and Modelling Topical Group). This model has also been applied to analyse a variety of experiments using mostly electron heat modulation on JET, ASDEX-Upgrade and FTU. The thresholds are found to be in the expected domain for micro-instabilities in tokamaks. The stiffness factor is found to cover a broad range of variation. One possible cause is the ratio of electron to ion temperature, which is found to exhibit a correlation with the stiffness factor.