1145
LETTERS TO THE EDITOR
A Criterion for Low Birthweight From IAN T RUSSELL* AND ALLEN J WILCOXf
In contrast, the proposal by Chen et al[ appears complex and arbitrary. In the example they cite, the advantages of the 1.5th centile rather than the first centile, the second centile or the 2.5th centile (of which the last is essentially equivalent to our proposal5) are not clear, as they themselves concede.' Moreover, their assertion that the criterion for low birthweight should maximise the number of residual births classified as of low birthweight, while minimising the number of predominant births so classified, lacks justification. In common with numerous authors since YIppo,6 we believe that this criterion should identify any birth whose weight implies a high risk of perinatal mortality.
•Health Services Research Unit, University of Aberdeen, Foresterhill, Aberdeen AB9 2ZD, UK. tDivision of Biometry and Risk Assessment, National Institute of Environmental Health Sciences, Research Triangle Park, NC 27709, USA.
The main problem with the inflexible criterion of 2500 g is that it identifies very different proportions of such births in different populations. We judge that, in the present state of knowledge, our proposed criterion of two standard deviations below the mean of the predominant Gaussian distribution5 represents the best compromise between practical utility and theoretical efficiency. Finally, Chen et al' misunderstand the first of our papers2 when they suggest that our 'approach implies that the criterion for low birthweight may be defined at the point of truncation'. On the contrary, we have always regarded the truncation point as a statistical device that is useful only in deriving the best estimates of the parameters of the predominant Gaussian distribution. For this reason, the last three of our papers3"5 do not mention the truncation point.
REFERENCES 1
2
3
4
5
6
Chen R, Wax Y, Lusky A, Toppelberg G, Barell V. A criterion for a standardized definition of low birthweight. Int J Epidemiol 1991; 20: 180-86. Wilcox A J, Russell I T. Birthweight and perinatal morality: I. On the frequency distribution of birthweight. Inl J Epidemiol 1983; 12: 314-18. Wilcox A J, Russell I T. Birthweight and perinatal mortality: II. On weight-specific mortality. Int J Epidemiol 1983; 12: 319-25. Wilcox A J, Russell I T. Birthweight and perinatal mortality: II Towards a new method of analysis. Int J Epidemiol 15: 188-95. Wilcox A J, Russell I T. Why small black infants have a lower mortality than small white infants: the case for population-specific standards for birthweight. J Pediatrics 1991; 116: 7-10. YIppo A. Das Wachstum der Frugeborenen von der Geburt bis zum Schulalter. Z Kindehielkd 1919; 24: 111-178.
Sunlight and Breast Cancer Incidence in the USSR From HAROUTUNE K ARMENIAN Sir—The paper by Gorham et al1 illustrates some of the problems with ecological studies. Although the authors identify correlations between breast cancer incidence and sunlight, there is a number of other factors, as plausible as sunlight, that could have provided Department of Epidemiology, School of Hygiene and Public Health, 615 North Wolfe Street, Baltimore, Maryland 21205, USA.
us with such positive correlations if data for such variables have been examined. For example, according to the reported distribution, one could as easily establish associations between ethnic and religious factors and the incidence of breast cancer in the Soviet Union. These being factors that are themselves very closely linked with marital, reproductive and behavioural factors.
Downloaded from http://ije.oxfordjournals.org/ at East Carolina University on July 13, 2015
In a recent issue of this journal, Chen et al' describe an extension of our approach to the analysis of birthweight and mortality;2^* to replace the existing inflexible criterion of 2500 g for low birthweight, they derive alternative criteria from the estimated parameters of the predominant Gaussian distribution of birthweight. In principle, we fully support the proposal that the criterion for low birthweight should reflect the birthweight distribution of the population under review. Indeed, we have proposed elsewhere5 that the criterion should be set two standard deviations below the mean of the predominant Gaussian distribution. This proposal of ours has the advantages of simplicity and consistency.
1146
INTERNATIONAL JOURNAL OF EPIDEMIOLOGY
It is dangerous to do ecological studies where the units of observation are as heterogeneous as the republics of the Soviet Union. There is a lot of variation as to sunlight exposure within a single large republic like the Russian Federation or the Ukraine, and to place those large republics and the Small Baltic or Transcaucasian republics within the same correla-
tional analysis with the same weights could definitely lead us to artifactual observations,
REFERENCE l Gorham E D, Garland F C, Garland C F. Sunlight and breast cancer in the USSR, int J Epidemiol 1990; 19: 820-24.
Downloaded from http://ije.oxfordjournals.org/ at East Carolina University on July 13, 2015
LETTERS TO THE EDITOR
A Criterion for Low Birthweight From IAN T RUSSELL* AND ALLEN J WILCOXf
In contrast, the proposal by Chen et al[ appears complex and arbitrary. In the example they cite, the advantages of the 1.5th centile rather than the first centile, the second centile or the 2.5th centile (of which the last is essentially equivalent to our proposal5) are not clear, as they themselves concede.' Moreover, their assertion that the criterion for low birthweight should maximise the number of residual births classified as of low birthweight, while minimising the number of predominant births so classified, lacks justification. In common with numerous authors since YIppo,6 we believe that this criterion should identify any birth whose weight implies a high risk of perinatal mortality.
•Health Services Research Unit, University of Aberdeen, Foresterhill, Aberdeen AB9 2ZD, UK. tDivision of Biometry and Risk Assessment, National Institute of Environmental Health Sciences, Research Triangle Park, NC 27709, USA.
The main problem with the inflexible criterion of 2500 g is that it identifies very different proportions of such births in different populations. We judge that, in the present state of knowledge, our proposed criterion of two standard deviations below the mean of the predominant Gaussian distribution5 represents the best compromise between practical utility and theoretical efficiency. Finally, Chen et al' misunderstand the first of our papers2 when they suggest that our 'approach implies that the criterion for low birthweight may be defined at the point of truncation'. On the contrary, we have always regarded the truncation point as a statistical device that is useful only in deriving the best estimates of the parameters of the predominant Gaussian distribution. For this reason, the last three of our papers3"5 do not mention the truncation point.
REFERENCES 1
2
3
4
5
6
Chen R, Wax Y, Lusky A, Toppelberg G, Barell V. A criterion for a standardized definition of low birthweight. Int J Epidemiol 1991; 20: 180-86. Wilcox A J, Russell I T. Birthweight and perinatal morality: I. On the frequency distribution of birthweight. Inl J Epidemiol 1983; 12: 314-18. Wilcox A J, Russell I T. Birthweight and perinatal mortality: II. On weight-specific mortality. Int J Epidemiol 1983; 12: 319-25. Wilcox A J, Russell I T. Birthweight and perinatal mortality: II Towards a new method of analysis. Int J Epidemiol 15: 188-95. Wilcox A J, Russell I T. Why small black infants have a lower mortality than small white infants: the case for population-specific standards for birthweight. J Pediatrics 1991; 116: 7-10. YIppo A. Das Wachstum der Frugeborenen von der Geburt bis zum Schulalter. Z Kindehielkd 1919; 24: 111-178.
Sunlight and Breast Cancer Incidence in the USSR From HAROUTUNE K ARMENIAN Sir—The paper by Gorham et al1 illustrates some of the problems with ecological studies. Although the authors identify correlations between breast cancer incidence and sunlight, there is a number of other factors, as plausible as sunlight, that could have provided Department of Epidemiology, School of Hygiene and Public Health, 615 North Wolfe Street, Baltimore, Maryland 21205, USA.
us with such positive correlations if data for such variables have been examined. For example, according to the reported distribution, one could as easily establish associations between ethnic and religious factors and the incidence of breast cancer in the Soviet Union. These being factors that are themselves very closely linked with marital, reproductive and behavioural factors.
Downloaded from http://ije.oxfordjournals.org/ at East Carolina University on July 13, 2015
In a recent issue of this journal, Chen et al' describe an extension of our approach to the analysis of birthweight and mortality;2^* to replace the existing inflexible criterion of 2500 g for low birthweight, they derive alternative criteria from the estimated parameters of the predominant Gaussian distribution of birthweight. In principle, we fully support the proposal that the criterion for low birthweight should reflect the birthweight distribution of the population under review. Indeed, we have proposed elsewhere5 that the criterion should be set two standard deviations below the mean of the predominant Gaussian distribution. This proposal of ours has the advantages of simplicity and consistency.
1146
INTERNATIONAL JOURNAL OF EPIDEMIOLOGY
It is dangerous to do ecological studies where the units of observation are as heterogeneous as the republics of the Soviet Union. There is a lot of variation as to sunlight exposure within a single large republic like the Russian Federation or the Ukraine, and to place those large republics and the Small Baltic or Transcaucasian republics within the same correla-
tional analysis with the same weights could definitely lead us to artifactual observations,
REFERENCE l Gorham E D, Garland F C, Garland C F. Sunlight and breast cancer in the USSR, int J Epidemiol 1990; 19: 820-24.
Downloaded from http://ije.oxfordjournals.org/ at East Carolina University on July 13, 2015
