Department of Epidemiology and Medical Statistics, School of Public Health, Postfach 100131, 33501 Bielefeld, Germany.
SirScherb et al.1 reported results of a statistical analysis on still birth proportions in three European regions for the years 19801992 and explore whether the decreasing time trend in these proportions was distorted during or after the Chernobyl accident.
I have serious reservations about the scientific validity of the paper.
First, both the introduction and the discussion include much material that is not pertinent to the investigation that leads to a confused discussion. The major objective of the authors was to verify their hypothesis that the radiation exposure in Germany due to the Chernobyl accident led to an increase in stillbirth proportion. Using data from other Western or Central (for the very specific definition of Western and Central see Scherb et al.) European countries their hypothesis could not be confirmed. Additionally, in Table 7 results' and conclusions' are listed that do not emerge from the analysis but are cited from former published and unpublished works of the authors.
Secondly, there are limitations in the statistical analysis:
|
|
|
The authors emphasize that causal inference can be drawn from this ecological' study, yet the discussion in Table 7 is centred around the radiation hypothesis'. Additionally, only parts of what is listed as results' in the table are presented in the paper. For example:
An analysis of the highly aggregated data can neither prove nor disprove an effect between radiation exposure and stillbirth rates, especially in view of an exposure that is far below background radiation in most of the countries (maximum 0.7 mSv, most regions below 0.30.03 mSv, see Table 1 in Dolk et al.2) and that areas with very different exposures are combined for this ecological analysis. The authors state in the Abstract that Our results ... contradict the present radiobiological knowledge. Based on the data presented, I am unable to accept this conclusion.
References
1 Scherb H, Weigelt E, Brüske-Hohlfeld I. European stillbirth proportions before and after the Chernobyl accident. Int J Epidemiol 1999; 28:93240.[Abstract]
2 Dolk H, Nicols R and a EUROCAT Working Group. Evaluation of the impact of Chernobyl on the prevalence of congenital anomalies in 16 regions of Europe. Int J Epidemiol 1999;28:94148.[Abstract]
SirBlettner disputes our results1 on a number of points. We wished to investigate whether in otherespecially easternEuropean countries, the effects on stillbirth and perinatal death rates after the Chernobyl accident were similar to those observed in Germany.25 We observed that, in Hungary, Poland and Sweden, the overall effect on the stillbirth rate was even stronger than in Germany.
The validity of our grouping into West, Central, and East countries (a and b) was questioned. This grouping served two purposes: to demonstrate and estimate a possible global spatial-temporal detrimental effect on European stillbirth rates after the Chernobyl accident and to ensure sufficient statistical power that could not be obtained using only single countries. No such coarse grouping was required if we had finely stratified data on contamination in Europe as we had for Bavaria and the former GDR,3,4 but to our knowledge, such European data do not exist. Blettner's reference to Table 1 in the paper by Dolk et al. does not help, because the reported doses cover only a small proportion of the respective western and central European populations. For example, population coverage in the United Kingdom is only 61 710 births out of 759 041 births in 1986. No data at all are presented for eastern European countries. In view of the lack of detailed European exposure and perinatal data we decided to group the countries according to the obvious radial neighbourhood to Chernobyl criterion into East, Central, and West strata. The difficulties we met in data availability, stability of stillbirth definition in the time periods studied and presumed exposure contrasts should be recognized.
We did not study the sex ratio as Blettner presumes (c). Because the sex ratio contains no direct information on stillbirths, we analysed the stillbirth odds ratio for gender: (male stillbirth/male live birth)/(female stillbirth/female live birth). This is an appropriate measure for the statistical analysis of the association between sex and stillbirth. For several countries we were unable to obtain gender specific stillbirth data and to apply the original West, Central, and East strata to the gender specific analysis. For example, for Poland, we did not obtain the gender specific stillbirth data according to the 28-week definition, instead, we were supplied with the gender specific stillbirth data according to a 600 g definition. Therefore, we investigated the time trend of the stillbirth odds ratio for gender in Denmark, Germany, Hungary, Norway and Sweden combined as well as in Poland on its own. The trend of the stillbirth odds ratio for gender including the change-point in ordinary linear weighted regression in 1987 and the trend of the sex ratio, for comparison, are shown in Figure 1 below. This effect can also be seen if countries are evaluated on their own or combined differently. It is interesting to note that stillbirth, unstratified by gender, in Denmark, Germany, Hungary, Norway, and Sweden combined shows essentially the same behaviour as in our original eastern group. Only the effect in 1986 is smaller and nonsignificant. It is not justified to imply that only certain combinations of the data would yield positive and significant results.
In contrast to Blettner (d), we do not consider that it is appropriate to use the same trend parameters for the three parts of Europe because the trends vary and differ highly significantly from each other. Moreover, applying a common trend model, i.e. only one intercept and only one slope, actually leads to a large overdispersion: Deviance/d.f. = 79.54. Using different intercepts and only one slope, improves the fit and renders the effects in the eastern part of Europe for 1986, 1987 and 19881992 positive and significant, similar to the result in our paper. Blettner ignored (1) our description of the relatively large and abrupt improvements in the stillbirth rates in western European countries in 1987 and 1988 (not 1986 and 1987 as Blettner assumed).
It was shown2,4,5 that the perinatal death rate in 1987 is elevated in Germany. As far as we know, it has never been claimed that the stillbirth rate in 1986 was significantly elevated in Germany. On the contrary, in Table 71 we report that, as a rule, stillbirths are not significantly elevated in western and central countries in 1986 in contrast to some eastern countries. Assuming a size of effect and power, we acknowledge that an increase in stillbirth rate in Germany is not likely to be detected.
Limitations of space did not allow account of the 20 countries involved and only significant change-points in the trends of the stillbirth odds ratios for gender could be mentioned. Hopefully, our work will serve as a starting point for further analyses because it is impossible to exhaust the topic in one paper. Figure 2 shows the trends in our four eastern European countries. Blettner is correct in saying that these trends are different, but so are trends of sub-regions of single countries. In our view, the variability between and within these countries is not so high as to preclude an analysis of the combined trend (an exception is the highly overdispersed Ukrainian data, see Figure 3
and Table 1
below). In Figure 2
and Figure 3
we also included simple linear logistic change-point models with the change-point in 1986. This is a parsimonious and impartial approach. Table 1
contains quantitative information on these models. The change-point is non-significant for Greece, borderline (or one-sided significant) for Sweden, and significant for Poland and Hungary. The change-point is also significant for these four countries combined, which is in close agreement to the significant effects of the partial eastern model in our paper. Figure 3
shows the data for Belarus and Ukraine. Table 1
also contains information on the change-points for these data alone and combined with the other countries. Because of the highly variable Ukrainian data from 1980 to 1987 the change-point model does not fit the Ukrainian data alone or that of all six countries combined (Deviance/ d.f. = 15.65 and 18.28, respectively). The less dispersed Belarus data reveal a one-sided borderline significant change-point in 1986 and combining this data with Greece, Poland, Sweden and Hungary yields a significant change-point in 1986, which also closely agrees with our original analysis. Thus, Blettner's statement (2): Belarus and Ukraine show no increase during or after 1986 is incorrect. We reiterate the lack of a definition of stillbirth in Belarus and Ukraine and all analyses involving these data are speculative. We interject here that we failed to point out in our paper, the highly significant effect in 1987 in the six eastern countries combined. We only mentioned the effect in 1986.
|
Blettner criticizes our interpretation in the abstract, which sounds somewhat contradictory to the simultaneously emphasized general limitations in our paper of results based on aggregated data. However, our statistical conclusion is conditional on the biological possibility that the observed spatial-temporal effects in Germany and Europe on small-scale and large-scale levels could be explained by radioactivity. Our position is strongly supported by the significant relative risks of stillbirth per 1 kBq/m2 on a district level in Bavaria3,5 (96 districts) and in the former GDR4 (197 districts) in 1987/1988 of 1.0072 (P = 0.002) and 1.0264 (P = 0.003), respectively.
Notes
GSFForschungszentrum für Umwelt und Gesundheit, Institut für Biomathematik und Biometrie, Postfach 1129, D-85758 Oberschleißheim, Germany. E-mail: scherb{at}gsf.de
References
1 Scherb H, Weigelt E, Brüske-Hohlfeld I. European stillbirth proportions before and after the Chernobyl accident. Int J Epidemiol 1999;28: 93240.[Abstract]
2 Körblein A, Küchenhoff H. Perinatal mortality in Germany following the Chernobyl accident. Radiation and Environmental Biophysics 1997; 36:37.[ISI][Medline]
3 Scherb H, Weigelt E. Spatial-temporal logistic regression of the caesium contamination and the time trends in annual stillbirth proportions on a district level in Bavaria, 1980 to 1993. In: Friedl H, Berghold A, Kauermann G (eds). Proceedings of the 14th International Workshop on Statistical Modelling. Technical University Graz, Austria: Institute of Statistics, Technical University Graz, 1999, pp.64750.
4 Scherb H, Weigelt E. Manuskript zur Stellungnahme und zu den präsentierten Folien bei der Anhörung im Bayerischen Landtag. Bayerischer Landtag, Ausschuß für Landesentwicklung und Umweltfragen, 21. Sitzung, Anhörung zum Thema Auswirkungen von ionisierenden Strahlen und insbesondere von niedrigen Strahlendosen auf die menschliche Gesundheit, Protokoll, Bayerischer Landtag, München, 21 Oktober 1999, pp.8197. www.bayern.landtag.de/infothek/infothek_f.htm
5 Scherb H, Weigelt E, Brüske-Hohlfeld I. Regression analysis of time trends in perinatal mortality in Germany 19801993. Environmental Health Perspectives 2000;108:15965; [Online 10 January 2000].
6 Dolk H, Nichols R, and a EUROCAT working group. Int J Epidemiol 1999;28:94148.[Abstract]
SirScherb et al.1 published stillbirth data for Belarus and Ukraine but excluded them from the analysis because the authors were not provided with the definition of stillbirths. But since these two countries suffered the highest fallout from Chernobyl, the data are of great interest and should be studied, even though there might be doubts about the definition. Therefore I analysed the data for Belarus plus Ukraine given in Table 2, but using a simple linear trend model. I only introduced one dummy variable for the year 1987, the year following the Chernobyl accident, since in an earlier investigation I had found an effect on German perinatal mortality for 1987 alone.2 I also re-analysed the combined data for Poland, Sweden, Hungary and Greece, denoted East by Scherb et al. using the same model, and finally performed a combined regression of the two data sets to increase the test power.
Data
In conversations with physicians from a St Petersburg maternity hospital, I learned that the criterion for stillbirth was a birthweight greater than 1000 g in the former Soviet Union at this time. But I was warned against using official data before the time of Glasnost (1985), because of habitual underreporting then, so I used only the data for 198592 in my analysis for Belarus and Ukraine.
Method
I use a simple exponential trend model which is linear when the logarithm of stillbirth rates is used. For testing a possible deviation of the stillbirth rate in 1987 from the trend of the other years, a dummy variable d(87) is introduced. A one-sided t-test is used to find out whether the coefficient of d(87) is greater than zero.
Results
In spite of the reduced time span, the fit to the data from Belarus plus Ukraine shows a significant increase of stillbirth rate in 1987 (P = 0.0245). Also for the data East the increase in 1987 is significant (P = 0.0134). The time constants as well as the excess rates in 1987 agree for both data sets within the limits of error. Therefore a combined regression with individual parameters for the intercepts is performed, using a common time constant and a common coefficient for the excess rate in 1987. This model then reveals a highly significant increase in 1987 (P = 0.0006). The excess in 1987 translates to 614 extra stillbirths (95% confidence interval 361867) in the eastern countries taken together, i.e. Poland, Sweden, Hungary, Greece, Belarus and Ukraine. The result of this analysis thus confirms the main result obtained by Scherb et al.
In Figure 1, the logarithms of the stillbirth rates are plotted, because in a semi-logarithmic plot exponential functions appear as straight lines. The circles represent the stillbirth data East, the squares, the data for Belarus plus Ukraine. The two parallel lines show the result of the combined fit.
|
Notes
Umweltinstitut München, Munich, Germany
References
1 Scherb H, Weigelt E, Brüske-Hohlfeld I. European stillbirth proportions before and after the Chernobyl accident. Int J Epidemiol 1999;28:93240.[Abstract]
2 Koerblein A, Kuechenhoff H. Perinatal mortality following the Chernobyl accident. Radiation and Environmental Biophysics 1997;36:37.[ISI][Medline]