Grais Rebecca F, Rose Angela M C, Guthmann Jean-Paul
Epicentre, 8, rue Saint Sabin, 75011 Paris, France.
Emerg Themes Epidemiol. 2007 Jun 1;4:8. doi: 10.1186/1742-7622-4-8.
In two-stage cluster surveys, the traditional method used in second-stage sampling (in which the first household in a cluster is selected) is time-consuming and may result in biased estimates of the indicator of interest. Firstly, a random direction from the center of the cluster is selected, usually by spinning a pen. The houses along that direction are then counted out to the boundary of the cluster, and one is then selected at random to be the first household surveyed. This process favors households towards the center of the cluster, but it could easily be improved. During a recent meningitis vaccination coverage survey in Maradi, Niger, we compared this method of first household selection to two alternatives in urban zones: 1) using a superimposed grid on the map of the cluster area and randomly selecting an intersection; and 2) drawing the perimeter of the cluster area using a Global Positioning System (GPS) and randomly selecting one point within the perimeter. Although we only compared a limited number of clusters using each method, we found the sampling grid method to be the fastest and easiest for field survey teams, although it does require a map of the area. Selecting a random GPS point was also found to be a good method, once adequate training can be provided. Spinning the pen and counting households to the boundary was the most complicated and time-consuming. The two methods tested here represent simpler, quicker and potentially more robust alternatives to spinning the pen for cluster surveys in urban areas. However, in rural areas, these alternatives would favor initial household selection from lower density (or even potentially empty) areas. Bearing in mind these limitations, as well as available resources and feasibility, investigators should choose the most appropriate method for their particular survey context.
在两阶段整群抽样调查中，第二阶段抽样（选择群内的第一户家庭）所采用的传统方法既耗时，又可能导致对目标指标的估计出现偏差。首先，通常通过转动一支笔来从群中心选择一个随机方向。然后沿着该方向将房屋数到群的边界，接着随机选择一户作为首个被调查的家庭。这个过程有利于选择位于群中心的家庭，但很容易改进。在最近于尼日尔马拉迪进行的脑膜炎疫苗接种覆盖率调查中，我们在城市地区将这种选择首个家庭的方法与另外两种方法进行了比较：1）在群区域地图上使用叠加网格并随机选择一个交叉点；2）使用全球定位系统（GPS）绘制群区域的边界，并在边界内随机选择一个点。尽管我们每种方法仅比较了有限数量的群，但我们发现抽样网格法对实地调查团队来说是最快且最简便的，不过它确实需要该地区的地图。一旦能够提供充分的培训，随机选择一个GPS点也被发现是一种好方法。转动笔并数房屋到边界是最复杂且最耗时的。这里测试的两种方法代表了比转动笔更简单、更快且可能更可靠的替代方法，用于城市地区的整群抽样调查。然而，在农村地区，这些替代方法会有利于从低密度（甚至可能为空）区域选择初始家庭。考虑到这些局限性以及可用资源和可行性，调查人员应针对其特定的调查背景选择最合适的方法。
