Department of Environmental and Occupational Health, College of Medicine, National Cheng Kung University, Tainan, Taiwan.
BMC Public Health. 2011 Oct 20;11:820. doi: 10.1186/1471-2458-11-820.
Despite its limitations, ecological study design is widely applied in epidemiology. In most cases, adjustment for age is necessary, but different methods may lead to different conclusions. To compare three methods of age adjustment, a study on the associations between arsenic in drinking water and incidence of bladder cancer in 243 townships in Taiwan was used as an example.
A total of 3068 cases of bladder cancer, including 2276 men and 792 women, were identified during a ten-year study period in the study townships. Three methods were applied to analyze the same data set on the ten-year study period. The first (Direct Method) applied direct standardization to obtain standardized incidence rate and then used it as the dependent variable in the regression analysis. The second (Indirect Method) applied indirect standardization to obtain standardized incidence ratio and then used it as the dependent variable in the regression analysis instead. The third (Variable Method) used proportions of residents in different age groups as a part of the independent variables in the multiple regression models.
All three methods showed a statistically significant positive association between arsenic exposure above 0.64 mg/L and incidence of bladder cancer in men and women, but different results were observed for the other exposure categories. In addition, the risk estimates obtained by different methods for the same exposure category were all different.
Using an empirical example, the current study confirmed the argument made by other researchers previously that whereas the three different methods of age adjustment may lead to different conclusions, only the third approach can obtain unbiased estimates of the risks. The third method can also generate estimates of the risk associated with each age group, but the other two are unable to evaluate the effects of age directly.
尽管存在局限性，生态研究设计仍在流行病学中广泛应用。在大多数情况下，需要进行年龄调整，但不同的方法可能会得出不同的结论。为了比较三种年龄调整方法，我们以台湾 243 个乡镇的饮用水砷与膀胱癌发病率的关系研究为例进行了比较。
在该研究乡镇的十年研究期间，共发现 3068 例膀胱癌病例，其中男性 2276 例，女性 792 例。我们应用三种方法对同一数据集进行分析，分析时间段为十年。第一种（直接法）直接标准化以获得标准化发病率，然后将其作为回归分析的因变量。第二种（间接法）间接标准化以获得标准化发病比，然后将其作为回归分析的因变量。第三种（变量法）将不同年龄组居民的比例作为多回归模型中的一个自变量。
三种方法均显示，男性和女性中砷暴露量大于 0.64mg/L 与膀胱癌发病率之间存在统计学上的正相关，但对于其他暴露类别，观察到的结果不同。此外，对于同一暴露类别，不同方法得到的风险估计值均不同。
本研究通过实证案例证实了其他研究人员之前提出的观点，即尽管三种不同的年龄调整方法可能会得出不同的结论，但只有第三种方法可以获得风险的无偏估计。第三种方法还可以生成与每个年龄组相关的风险估计值，但其他两种方法无法直接评估年龄的影响。
