In this article, we consider technological or industrial equipments that are subject to degradation. These systems undergo maintenance actions, which reduce the degradation level. Several models have already been proposed for this type of degradation. Here we focus on the ARD 1 model (Arithmetic Reduction of Degradation). We study the estimation of its parameters. We will give four different approaches according to the way the degradation levels are observed. In the first case, we assume that we observe the degradation levels just before and after each maintenance. In the second (resp. third) case, we observe the degradation level just before (resp. after) maintenance but not after (resp. before). In the last case, we do not observe the degradation level neither before nor after the maintenances. Each case leads to a different writing of the likelihood of the observations and therefore to different estimations.