 Each of the N units undergoes exactly one of D interventions at time step T_0, with 1 \le T_0 < T, prior to which all units experience no intervention, i.e., control.
<clarity>  Each of the N units undergoes exactly one of D interventions at time step T_0, with 1 \le T_0 < T, prior to which all units experience no intervention, i.e., control.
 Each of the N units undergoes exactly one of D interventions at time period T_0, with 1 \le T_0 < T, prior to which all units experience no intervention, i.e., control.