# Impact of Urbanization and Sunlight Exposure on Cataract Incidence
## 2 Materials and Methods  
The primary data source of this study was the Taiwan National Health Insurance Research Database (NHIRD). We chose the Longitudinal Health Insurance Database 2010 (LHID2010), a subset of NHIRD data, representing 1,000,000 randomly sampled beneficiaries insured in 2010 from National Health Insurance (NHI). Statisticians have confirmed that there is no significant difference in age, gender, or healthcare costs from the entire population, which consists of all beneficiaries under the NHI program.  
This research focused on senile cataract (ICD-9 code: 3661, including ICD-9 codes 36610-36619) but excluded drug-induced cataract, congenital cataract, metabolic cataract, complicated cataract, and traumatic cataract. Therefore, the case group would be age 40 and above and diagnosed as senile cataract but would exclude some specific diagnoses mentioned above; the control group would be age 40 and above without a diagnosis of senile cataract.  
There were a total of 1,094,727 insured in the LHID2010 database. After excluding those aged 39 and below, there were a total of 396,321 insured included in our analysis. Among those included in the research, 3080 were diagnosed as senile cataract in the case group, 393,241 were in the control group. In terms of sunlight exposure duration, we obtained the data from 28 meteorological stations across Taiwan. We collected the actual daily sunny hours for a consecutive 132 months from January of 2001 to December of 2011 and then calculated average monthly sunlight hours for each location.  
As to urbanization levels in Taiwan, the 368 townships in Taiwan were initially classified into seven urbanization levels, according to research by the Taiwan National Health Research Institutes. The levels of urbanization were determined by several factors, including population density, population ratio of the elderly (over 65), population ratio of farmers, and physicians per 100,000 populations, etc. The composite indicator of urbanization in Taiwan was established using the above factors through cluster analysis. Therefore, it is not easy to present the accurate range of each factor. However, Table 1 shows the rough range of major factors in establishing levels of urbanization in Taiwan [20].  
To simplify the analysis, we modified and reclassified the seven urbanization stratifications of Taiwan townships into five levels in this study: highly urbanized, high urbanized, medium urbanized, low urbanized, and remote areas.  
We performed a logistic regression controlled by confounding variables described above to see the impact of different urbanization levels on the incidence of cataracts (Figure 1).  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline  & **Population** & **Population** & **Population** & **Physicians** \\  & **Density** & **Ratio of Elderly** & **Ratio of** & **per 100,000** \\  & **(per KM2)** & **(over 65)** & **Farmers** & **Populations** \\ \hline Highly urbanized & 23,481–26,101 & 16.52–21.73\% & 0.15–0.85\% & 217–528 \\ High urbanized & 1158–2362 & 12.47–15.47\% & 8.16–13.13\% & 243–345 \\ Medium urbanized & 1008–2583 & 12.97–14.31\% & 10.58–10.88\% & 158–230 \\ Low urbanized & 452–952 & 16.51–17.49\% & 10.82–19.41\% & 124–137 \\ Remote areas & 5–11 & 9.44–18.99\% & 31.36–32.13\% & 24–83 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Major factors measuring urbanization in Taiwan and their rough ranges [20].  
Figure 1: Conceptual Framework.