The methods provide for detecting at least one operational parameter of a functional unit of the plant and calculating a current value of at least a first statistical function of the operational parameter in a current temporal window, the current value of the first statistical function defining a first coordinate of a point of current operation of the functional unit. A step is also provided of verifying whether the point of current operation is within a range of allowable values of the first statistical function, the values contained in the range of allowable values corresponding to a correct operation of the functional unit. In case the point of current operation is outside the range of allowable values, the position is determined of the point of current operation with respect to the range of allowable values and a statistical diagnosis is provided of the cause of the deviation of the current value from the range of allowable values based on the position.