Response

Yiing-Jenq Chou1 and Nicole Huang2

1 Department of Social Medicine, National Yang Ming University, Taipei, Taiwan
2 Department of Health Policy and Management, Johns Hopkins Bloomberg School of Public Health, Baltimore, MD, USA

Correspondence: Dr Yiing-Jenq Chou, Department of Social Medicine, School of Medicine, National Yang Ming University, 155 Ni-Long Street, Taipei, Taiwan 112. E-mail: yjchou{at}ym.edu.tw

We agree with Dr Lu1 that more research, not limited to individual-level studies, is required to assess the impacts of earthquake on suicide. The role of the physical and social environment should not be overlooked.2–4 Ecological and multilevel studies can help contribute to this line of research. Individual and ecological studies are different analyses, but of equal importance.

Dr Lu's assertion1 that we ‘explicitly stated that our individual-level study was better than previous ecological disaster studies’5 is not entirely correct. The actual statement in our Discussion section was that ‘the accuracy of our study is relatively higher than that of some previous ecological disaster studies’ because previous disaster studies on suicides seldom differentiate victims from non-victims at the individual level, and incorporate relatively comprehensive socio-economic and health status information on each member of the study population due to lack of individual-level data.5 We, by no means, claim that our study is better than all previous ecological disaster studies. We believe there is always more than one ‘correct’ way to address one question. No one design is better than the other.

We disagree with Lu's argument that failure to include contextual variables into the analyses is the main reason why we only observed a small earthquake impact on suicide.1 Lu totally ignores our area-level analyses between earthquake affected and unaffected municipalities. First, in our study, we did compare crude and adjusted suicide rates between earthquake affected and unaffected municipalities before and after the earthquake (ref. 5, Tables 2–4). Our study5 includes all 68 municipalities in central Taiwan. The earthquake mainly struck the 22 municipalities on the east side of this region, and left the 46 municipalities on the west side virtually unaffected. The central government provided national disaster assistance to these 22 municipalities, referred to in our study as the ‘affected’ area. The other 46 municipalities are defined as the ‘unaffected’ area, and serve as the control for the pre–post area-level comparison. Instead of using a categorical variable to classify municipalities by degree of post-quake damage, we used a dichotomous variable to obtain sufficient statistical power. Classifying municipalities into multiple categories by degree of earthquake damage may be more informative, but it could lead to a substantial loss of statistical power due to smaller sample sizes within each category when studying a rare event like suicide. The non-significant association between degree of damage and suicide rates presented in Lu's analysis provides a strong support for this point.1 Second, in addition to area affected status, urbanization of residential area was also included in our regression analyses as a contextual variable. Third, our findings, shown in Table 2 of our paper, clearly demonstrated that crude suicide rates increased moderately in both affected and unaffected areas after the earthquake.5 The crude suicide rate in the affected area increased slightly more than that in the unaffected area after the earthquake. After stratification by victim status, the post-quake suicide rate among victims was much higher than that among non-victims in the same area. The higher suicide rate among victims, and the higher proportion of victims residing in the affected area had significant influence on the suicide rate differentials between affected and unaffected areas before and after the earthquake. Thus, individual victim status was apparently an important mediating variable in the relationship between earthquakes and suicides. The univariate and multivariate results in our paper (ref. 5. Tables 3–4) confirmed the significant and independent relationship between individual victim status and suicide.5 The decrease in odds ratio from 1.71 to 1.46 observed between individual victim status and suicide after controlling for two contextual variables and others provides opposing evidence to Lu's speculation.1,5

Furthermore, the results illustrated in Lu's letter are misleading.1 First, the 1999 Taiwan earthquake not only severely impacted Nantou county, but also Taichung county. Including only Nantou in his analysis might lead to a serious selection bias. Second, the 95% CI of the standardized mortality ratio (SMR) estimates were very wide and implied imprecision of these estimates. The wide interval ranges were likely to be caused by smaller sample sizes within categories. Investigators have to be careful in making a trade-off between sample sizes/statistical power and number of variable categories, especially when studying a rare event like suicide. Third, Lu used Hsienyi as an example to illustrate huge area variations in SMR. However, the example is misleading. It is generally known that Hsienyi and Jenai are two atypical municipalities in Taiwan as a result of their unique geographical location and ethnic population. Unlike the other 11 municipalities in Nantou, Hienyi and Jenai are located in remote mountainous regions and the population is mainly aboriginal. According to previous research,6–9 aborigines have higher suicide rates than non-aborigines. We believe that the unique ethnic composition is the most likely explanation for the exceptionally higher SMR observed in these two municipalities Excluding these two atypical municipalities, the actual variation in suicides rates between municipalities becomes much smaller.

Lu refers to the fact (Figure 1) that both Nantou and Taiwan showed an increase in suicide rates after the earthquake, and Nantou had a slightly faster increase than Taiwan after the earthquake, which is consistent with our findings (Table 2).5 Overall, Lu raises an interesting argument, but fails to present sufficient evidence to demonstrate significant variation in suicide rates between areas with varying degree of damage before and after the earthquake. Small sample sizes, selection biases, and possible confounding factors are the major limitations of his analyses.

In our study, we tried to assess impacts of earthquake on suicide and found that, holding area and other individual characteristics constant, earthquake victims had a significantly higher risk of post-quake suicide than non-victims. Individual victim status is an important explanatory factor in understanding the impact of earthquake on suicide. Every study has its limitations, so does our study. We would encourage other investigators, including critics of our findings, to conduct different (contextual, individual, or multilevel) and scientifically sound studies to confirm or refute our findings.


    References
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 References
 
1 Lu TH. Earthquake and suicide: bringing context back into disaster epidemiological studies. Int J Epidemiol 2004; 33:1406–09.[Free Full Text]

2 Macintyre S, Ellaway A. Ecological approaches: rediscovering the role of the physical and social environment. In: Berkman LF, Kawachi I (eds). Social Epidemiology. New York: Oxford University Press, 2000, pp. 332–48.

3 Schwartz S. The fallacy of the ecological fallacy: the potential misuse of a concept and the consequences. Am J Public Health 1994;84:819–24.[Abstract]

4 Susser M. The logic in ecological: I. the logic of analysis. Am J Public Health 1994;84:825–29.[Abstract]

5 Chou YJ, Huang N, Lee CH et al. Suicides after the 1999 Taiwan earthquake. Int J Epidemiol 2003;32:1007–14.[Abstract/Free Full Text]

6 Wen CP, Tsai SP, Shih YT, Chung WS. Bridging the gap in life expectancy of the aborigines in Taiwan. Int J Epidemiol 2004;33:320–27.[CrossRef][ISI][Medline]

7 Lee CS, Chang JC, Cheng AT. Acculturation and suicide: a case-control psychological autopsy study. Psychol Med 2002;32:133–41.[ISI][Medline]

8 Cheng AT, Mann AH, Chan KA. Personality disorder and suicide. A case-control study. Br J Psychiatry 1997;170:441–46.[Abstract]

9 Cheng AT. Mental illness and suicide. A case-control study in east Taiwan. Arch Gen Psychiatry 1995;52:594–603.[Abstract]





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