1. A diagnostic method for a technical installation for determining a cause of a fault event described by a fault state variable, the method which comprises: establishing an operating state of the installation defined by state variables, by determining diagnostic parameters each characterizing one of the state variables; compiling a dependency tree containing at least some of the diagnostic parameters by configuring the dependency tree with the following hierarchical levels numbered Hn, where n&equals;1, 2,... : a first hierarchical level containing a fault diagnostic parameter characterizing a fault state variable, a second hierarchical level, following the first hierarchical level, and containing diagnostic parameters defining state variables acting directly on the fault state variable, a hierarchical level Hn&plus;1, following a given hierarchical level Hn, and containing diagnostic parameters defining state variables acting directly on the state variables characterized by the diagnostic parameters at the given hierarchical level Hn; determining a fault path in the dependency tree starting from the fault diagnostic parameter, by identifying a responsible diagnostic parameter at the hierarchical level Hn&plus;1 depending on one of a magnitude and a change direction of a diagnostic parameter at the given hierarchical level Hn; and inferring a cause of the fault from the responsible diagnostic parameter identified in the identifying step at a last one of the hierarchical levels.