1. A method for monitoring degradation of an onboard device of an aircraft during functioning thereof, implemented by a computer, the degree of degradation of the onboard device being defined by an abnormality score formed by the counting of occurrences of abnormalities noted by a control system of the device during functioning thereof, the monitoring method comprising a step of comparing an abnormality score obtained for an observation sequence of given length (n, t) with a decision threshold (ks) and a step of issuing an alarm in the event of the decision threshold (ks) being reached or exceeded, the decision threshold (ks) being determined automatically for a given probability of alarm Pa, corresponding to the probability that an alarm will be issued during the monitoring method whereas the onboard device is sound, by means of the following steps: a step of obtaining an abnormality score (r) on at least one reference sequence corresponding to flights of the aircraft without degradation and with a length (m, tc) equal to a plurality of lengths (n, t) of observation sequences; a step of adjusting a discrete probability law making it possible to find the abnormality score (r) obtained on the reference sequence; a step of calculating the decision threshold (ks) such that, by applying the adjusted discrete probability law during the previous step to an observation sequence having the given length (n), the probability of an abnormality score higher than or equal to the decision threshold (ks) occurring a number S of times for N consecutive sequences is less than an elementary probability Peac of exceeding of a threshold evaluated from the given alarm probability Pa, N and S being able to be equal to one, in which the elementary probability Peac of exceeding of a threshold being evaluated from the given alarm probability Pa by means of the formula: 
 Pcac=□ (k,fi·k+1) −1 (Pa)Peac= B S,N−s−1 −1 (Pa) in which N is the number of observation sequences, S the number of times where the threshold ks will have been exceeded for N successive sequences, and B S,N−s−1 −1 the inverse Euler beta distribution function of parameters S and N−S+1.