Find the minimum value within grouped data

I don't think I have this 100% right but we might be close.

This returns a table of incidents showing the duration between the first datetime of the incident and the duration of each step in minutes for each step for each subsequent step.

Table = 

VAR T1 = SUMMARIZE(
             FILTER(
                 'Incidents',
                 LEFT([ActionDescription],20)="Incident assigned to"
                 ),
                 Incidents[IncidentNo],
                 "First Assigned",MIN('Incidents'[ActionDateTime])
                 )
                 
                 
RETURN ADDCOLUMNS(
            NATURALINNERJOIN('Incidents',T1),
            "Duration",IFERROR(DATEDIFF([First Assigned],'Incidents'[ActionDateTime],MINUTE) ,0)      



 )