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Different Convergence Parameters Applied to the S-PLUS GAM Function

Katsouyanni, Klea; Touloumi, Giota; Samoli, Evangelia; Gryparis, Alexandros; Monopolis, Yannis; LeTertre, Alain; Boumghar, Azedine; Rossi, Giuseppe; Zmirou, Denis; Ballester, Ferran; Anderson, Hugh Ross; Wojtyniak, Bogdan; Paldy, Anna; Braunstein, Rony; Pekkanen, Juha; Schindler, Christian; Schwartz, Joel

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Department of Hygiene and Epidemiology

University of Athens Medical School

75, Mikras Asias Street

115 27 Athens, Goudi GREECE

(address correspondence to: Klea Katsouyanni)


Institut de Veille Sanitaire

Paris, France

Fisiologia Clinica CNR

Unit of Epidemiology and Biostatistics

Pisa, Italy


Nancy, France

Epidemiology and Statistics Unit

Escola Valenciana d‘Estudis per a lat Salut

Conselleria de Sanitat

Generalitat Valenciana

Valencia, Spain

Department of Public Health Sciences

St. George’s Hospital Medical School

University of London

London, United Kingdom

National Institute of Hygiene

Department of Medical Statistics

Population Studies Laboratory

Warsaw, Poland

J. Fodor National Public Health Centre

National Institute of Environmental Health

Division of Environmental Health Assessment

Budapest, Hungary

Department of Epidemiology and Preventive Medicine

Tel Aviv University

Tel Aviv, Israel

National Public Health Institute

Unit of Environmental Epidemiology

Kuopio, Finland

Universitat Basel

Institut fur Sozial-und Praventivmedizin

Basel, Switzerland

Department of Environmental Health

Harvard School of Public Health

Boston, MA

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To the Editor:

Recently the Health Effects Institute (HEI) circulated a letter from the investigators at the Johns Hopkins Bloomberg School of Public Health who conducted the National Morbidity, Mortality and Air Pollution Study (NMMAPS) on short-term effects of air pollution on health, in which they report a problem with the default convergence criteria in the S-PLUS GAM function they have been using. Applying more stringent convergence criteria resulted in reducing the mortality increase associated with an increase of 10 μg/m3 in PM10 concentrations from 0.44% to 0.22%.

In this report questions were raised about the effect of this problem on the results of other similar studies. The European multicenter project Air Pollution and Health: A European Approach (APHEA), with objectives similar to the NMMAPS, has published results on particulate matter effects on total mortality in a recent issue of Epidemiology, 1 using the S-PLUS GAM function with default convergence criteria for some of the analyses.

We have reanalyzed our data using more stringent convergence criteria as proposed by the Johns Hopkins Group (specifically, the max number of iterations was set to 1,000 and the difference of two successive coefficients to 10−14). We found that the change in the estimated effect was marginal. The estimated combined increases in mortality associated with an increase in 24-hour PM10 (particulate matter less than 10 μm in aerodynamic diameter) and black smoke (BS) concentrations by + 10 μg/m3 were reduced by 4% and remain identical when reported with one significant digit; for PM10 the increase in mortality was 0.6% (95% confidence interval = 0.4%–0.8%) and for BS the increase was 0.6% (0.3%–0.8%). Tables showing individual city results using both the default and the more stringent convergence parameters are available with the electronic version of this letter at http://www.epidem.com.

It appears, therefore, that the extent of the bias reported in the NMMAPS is not necessarily applicable in all studies that use the GAM function. The reasons may lie in differences concerning the smooth functions introduced in the model, the number of the degrees of freedom, the lag times considered, the way by which confounders have been adjusted, or the data patterns. Small changes in the model-derived estimates are expected under any change in the modeling procedure and are, in fact, trivial in comparison with differences among individual city estimates within each project.

Optimization of model choice is a continuous procedure and the initiated collaboration between NMMAPS, APHEA and Canadian researchers will soon address further methodologic issues.

Klea Katsouyanni

Giota Touloumi

Evangelia Samoli

Alexandros Gryparis

Yannis Monopolis

Alain LeTertre

Azedine Boumghar

Giuseppe Rossi

Denis Zmirou

Ferran Ballester

Hugh Ross Anderson

Bogdan Wojtyniak

Anna Paldy

Rony Braunstein

Juha Pekkanen

Christian Schindler

Joel Schwartz

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1. Katsouyanni K, Touloumi G, Samoli E, et al. Confounding and effect modification in the short-term effects of ambient particles on total mortality: results from 29 European cities within the APHEA2 project. Epidemiology 2001; 12: 521–531.

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© 2002 Lippincott Williams & Wilkins, Inc.

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