float calculateGlobalPersistence(){    	  
    	  int disappearanceCount = 0;
    	  for (int i=0;i<this.persistence.size();i++){
    		  if (this.persistence.get(i)==0) disappearanceCount++;
    	  }    	 
    	  return (float)(this.numTimeSlices - disappearanceCount)/this.numTimeSlices;
      }