Figure 6 .

Extension of the sampling frame concept to three dimensions produces a “brick” in the shape of a rectangular prism (dashed outline). The brick is bordered by exclusion planes (labeled “exclude”) on three sides. Objects intersecting the bottom of the brick are excluded, as are objects intersecting the back or left side of the brick. In addition, there are exclusion regions adjacent to the front and top faces of the brick. The top, front, and right side of the brick are inclusion faces. The brick has height h perpendicular to the plane of sectioning.