Time
If the crop yield is to be measured in several successive seasons, then we can
either use the same sample area in all seasons or select new sample areas
each season. The former requires placing permanent markers in the field if
we are to return to the same quadrats in later years. If the objective is to
measure the system each season, then there is no advantage in keeping the
same quadrats over time. Indeed, if we are trying to estimate long-term
productivity higher precision will be obtained by selecting a new sample of