A learning method of one or more thresholds for one or more indicators of the operational state of a machine, such as an aircraft engine, calculated from signals provided by one or more sensors associated with the machine, comprising the following operation, implemented by a computerised processing unit: - acquiring signals characteristic of normal operation of the machine and of signals characteristic of abnormal operation of the machine; characterised in that it further comprises the following operations, implemented by the computerised processing unit: - for each of the signals characteristic of normal operation, calculating (SOUS) so-called deviation signals through subtraction of said signal characteristic of normal operation with each of the signals characteristic of normal operation other than said signal characteristic of normal operation and each of the signals characteristic of abnormal operation; - for each of the deviation signals, calculating an indicator (INDC-SOUS) comprising: - calculating a statistical moment of the deviation signal or the energy of the deviation signal; or - counting the number of points in the signal subtracted from a signal characteristic of normal operation that are present outside an envelope of said signal characteristic of normal operation; or - counting the number of peaks from amongst n peaks in the signal characteristic of normal operation which coincide with a peak from amongst p peaks of the signal subtracted from said signal characteristic of normal operation; - determining (CAL-TSH), from the deviation signal indicators, an indicator threshold allowing the deviation signals, calculated by subtracting a signal characteristic of normal operation with another signal characteristic of normal operation, to be distinguished from deviation signals calculated by subtracting a signal characteristic of normal operation with a signal characteristic of abnormal operation; - recording the indicator threshold in a reference database. - acquiring signals characteristic of normal operation of the machine and of signals characteristic of abnormal operation of the machine; characterised in that it further comprises the following operations, implemented by the computerised processing unit: - for each of the signals characteristic of normal operation, calculating (SOUS) so-called deviation signals through subtraction of said signal characteristic of normal operation with each of the signals characteristic of normal operation other than said signal characteristic of normal operation and each of the signals characteristic of abnormal operation; - for each of the deviation signals, calculating an indicator (INDC-SOUS) comprising: - calculating a statistical moment of the deviation signal or the energy of the deviation signal; or - counting the number of points in the signal subtracted from a signal characteristic of normal operation that are present outside an envelope of said signal characteristic of normal operation; or - counting the number of peaks from amongst n peaks in the signal characteristic of normal operation which coincide with a peak from amongst p peaks of the signal subtracted from said signal characteristic of normal operation; - calculating a statistical moment of the deviation signal or the energy of the deviation signal; or - counting the number of points in the signal subtracted from a signal characteristic of normal operation that are present outside an envelope of said signal characteristic of normal operation; or - counting the number of peaks from amongst n peaks in the signal characteristic of normal operation which coincide with a peak from amongst p peaks of the signal subtracted from said signal characteristic of normal operation; - determining (CAL-TSH), from the deviation signal indicators, an indicator threshold allowing the deviation signals, calculated by subtracting a signal characteristic of normal operation with another signal characteristic of normal operation, to be distinguished from deviation signals calculated by subtracting a signal characteristic of normal operation with a signal characteristic of abnormal operation; - recording the indicator threshold in a reference database.