DESIGN: Prospective, cross-sectional study.

设计：前瞻性的横向研究。

This research is a cross-sectional investigation.

此研究是一个横断面调查。

DESIGN: Cluster sampling and cross-sectional investigation.

设计：整群抽样，横断面调查。

DESIGN: Randomized cluster sampling, cross-sectional study.

设计：随机整群抽样，横断面调查。

Inversely proportional to the cross-sectional area of the rod.

棒子的横截面积，成反比。