Why was the paper-slip method used to select the grid segments (quadrats)? Individual choice will lead to selection of zones that represent the highest density which are more important to an accurate estimate.
Random sampling avoids selecting "outliers" that do not represent the actual population size.
Avoidance of areas that are difficult to count and should not be used in the overall average.
Selection of random quadrats to sample provided data that more represented the average square rather than that of a particular area.