ARTÍCULO ESPECIAL

 

Guidelines for the publication of manuscripts in Salud Pública de México*

 

 

In January 1978 a group of editors of some of the most popular medical journals (Annals of Internal Medicine, British Medical Journal, The Lancet, The New England Journal of Medicine and the Journal of the American Medical Association) met in Vancouver, Canada, and agreed to standardize the regulations for publication of biomedical articles. One year later, the journals American Review of Respiratory Diseases, British Medical Journal, The Lancet and Annals of Internal Medicine published the Vancouver Declaration: "New guidelines for the publication of manuscripts in biomedical journals." The editors of these and other medical journals -who in 1980 formed the International Committee of Medical Journal Editors- agreed to apply the regulations contained in this document to the manuscripts submitted to these journals for publication.1 In october 1981, the original document was revised and a second edition published. Since then, the Committee prepared various independent reports which were used in a third and fourth edition.2,3

In April 1987, Salud Pública de México was officially accepted in the group of journals which adopted the Vancouver agreements. Therefore, the design of its new guidelines, which follow, fundamentally respect the proposals of the above-mentioned document.

 

Objectives and general information

Salud Pública de México is a bi-monthly journal published by the Instituto Nacional de Salud Pública (National Institute of Public Health). Its general purpose is to disseminate information on public health, the latter being understood as the application of the biological, social and administrative sciences to the analysis of health phenomena in human population, as well as the identification of health needs and the organization of integrated services.4  The journal’s specific objectives are to review historical experiences and stimulate new currents of thought in the field of health issues; to participate in the debate over changes in health conditions and in the organized social response to such conditions; and to serve as a forum where all innovations in the field of public health and related areas can be discussed, with the greatest possible rigor.

Salud Pública de México is indexed in Current Contents/Social and Behavioral Sciences, Social Sciences Citation Index, Index Medicus, Index Medicus Latinoamericano, Excerpta Medica, Indice de Revistas Científicas Mexicanas de Excelencia del Consejo Nacional de Ciencia y Tecnología, Bibliomex Salud, Periódica, Indice de Revistas de Educación Superior e Investigación Educativa (IRESIE), and in the on-line systems Medline and Lilacs, as well as in CD Artemisa. At present, it prints 4 000 copies (4 500 for special issues) and it is distributed in Mexico and abroad among professionals and technicians in the field of public health.

 

Legal rights and general criteria for publication

Salud Pública de México reserves all copyrights in accordance with the terms of the legislation in force. All or part of the material published in the journal may be reproduced provided express permission has been obtained, in writing, from Salud Pública de México, and provided that the source is cited.

All proposed articles must be original and not previously published, and may not be submitted to any journal while under consideration by Salud Pública de México. Complete papers, previously published in summarized form, may also be accepted, or unpublished papers presented at conferences or seminars.

Signed articles are the sole responsibility of the author and do not necessarily reflect the opinions of the journal, of the National Institute of Public Health, nor of the institution to which the author is affiliated.

 

Specific criteria for publication and for the selection process

Salud Pública de México is a bilingual journal that publishes articles in Spanish and in English only on subjects of public health and related areas (Table 1). The journal does not undertake translations of the articles.

 

Table I
TOPICS OF SALUD PÚBLICA DE MÉXICO

Biomedical sciences

· Infectious diseases
- Gastrointestinal
- Respiratory
- Tuberculosis
- Cholera
- Dengue
- Oncocercosis
- Malaria
- Other parasitic diseases
- Tetanus
· Food intoxications
· Seroepidemiology
· AIDS and STD
· Virus and cancer
· Vaccines

Epidemiology

· Injuries
· Demography

· Addictions (alcohol, tobacco and drugs)
· Congenital diseases and perinatal complications
· Communicable diseases
· Chronic & degenerative diseases
- Cancer
- Cardiovascular diseases
- Diabetes
· National health surveys
· Epidemiology
· Vital statistics
· Nosocomial infections
· Morbidity
· Mortality
· Nutrition
· Environmental health
· Dental health
· Women's health
· Child health
· Aging
· Mental health

· Occupational health
· Reproductive health
· Seroepidemiology
· AIDS and sexually transmitted diseases
· Health transition
· Vaccines

Health systems

· Health care reform
· Administration
· Accessibility
· Primary health care
· Quality
· Coverage
· Costs
· Decentralization
· Resources availability
· Health economics
· Health education, community work
· Efficiency

· Medical employment
· National health surveys
· Equity
· Financing
· Health manpower training  and development 
· Public management in health
· Health information systems
· Organization
· Health policies
· Medical prescription
· Priorities in health
· Programming
· Human resources for health
· Regionalization
· Sanitary regulation
· International health
· Sociology of health
· Technology assessment
· Utilization

 

Criteria for selection include the articles’ scientific soundness and the originality, the topicality and timeliness of the information. In addition, the journal gives priority to original articles with research findings over reviews and essays.

For acceptance or rejection, each article is subjected to an evaluation process that includes:

· A first review, which remains in the hands of the editors, determines the manuscript’s importance, relevance and depth, and also whether or not it corresponds to the editorial line, follows the guidelines of  Salud Pública de México and complies with the general criteria for publication.
· A second review, which is entrusted to at least two experts. If should their opinions not agree, the article is submitted to the consideration of the Editorial Board or, alternatively, a third evaluation is requested.

The decision may be acceptance, rejection, or conditional acceptance subject to the changes suggested by the reviewers.

All decisions are communicated to the author in writing, within a period of two to three months from the date of receipt of the original manuscript. If the article is subject to conditional acceptance, the new version, both in printed copy and on a diskette, must be sent within the stated period, but in no case may it exceed 30 days.

 

Content of Salud Pública de México

The material sent to Salud Pública de México is published in the form of editorials, original articles, brief communications, review articles, essays, updates, classics, indicators, health news, reviews, letters to the editor, and articles on special topics. The journal also includes announcements of activities and scientific publications provided these are requested in writing. In exceptional cases, the journal accepts translations of works originally published in another language, if the subject-matter is of interest to Spanish-language readers.

Editorial. It is a section that expresses opinions. It consists of reflections on topical matters, comments about articles or about the journal itself. The title of the editorial should not be excessively long and complex, since the text itself is short. Generally, the use of subtitles is not recommended. The author’s institutional affiliation should be presented below the author’s name, which will appear at the end of the text. If references are used, they should follow the same format and style as for the articles.

Original articles. These include findings of original research projects. Original articles should be structured as follows: Introduction, material and methods, findings and discussion. At the beginning of each work, the journal presents the abstracts and key words, in both Spanish and English; the maximum number of tables and/or figures is five.

Brief communications. This section will include articles limited to five pages of text (or about 1 000 words), plus two tables and/or figures, and an abstract limited to 100 words. Such articles present preliminary or collateral findings that merit further research or that support decision-making in health services. Moreover, they will be evaluated in the context of local or current conditions of known studies. Presentation must comply with the same guidelines for content and rigorous scientific standards that apply to the longer articles.

Review articles. In this section, the journal includes critical reviews that will be divided according to the article’s content, but that will always include introduction and conclusions, as well as an abstract and key words in Spanish and in English; the number of tables and/or figures is five.

Essays. This section includes writings devoted to a critical examination and reflection on topics related to public health; they follow the same format and style as the review articles, but it is recommended that they should be shorter in length. Such essays are received when expressly requested by Salud Pública de México; however, those interested may send proposals, which will be reviewed by the Editorial Board to establish whether or not they are relevant.

Classics. This section reproduces articles that, owing to their influence in the field of public health, have become required references.

Indicators. These are current and relevant statistical information for the public-health area; they include epidemiological and health data and indicators, basically presented in graphic form through tables and figures. In this section, authorship is institutional rather than individual.

Health news. This section will report on the results of events that, because of their importance, may be of interest to the development of scientific knowledge.

Updates. Descriptive articles that are intended to present the elements that make it possible to know the state of the art of the research in a particular area of public health. They should include recent information offering the reader an updated overview of the area in question.

Public health pages. These are book reviews, comments, or brief notes on recent scientific developments in public health and related areas.

Letters to the editor. This section aims to stimulate a feedback process making it possible to evaluate and, if appropriate, redefine the editorial policy and the output of Salud Pública de México. This section also serves as a forum for the open discussion of topics dealt with in the journal and other matters of interest. The letters must contain title, text, institutional affiliation of the author and, if appropriate, references.

Articles on special topics. These are documents that do not fit into any of the above-mentioned categories, but, because of their content, are of interest to public health researchers and academics. Such documents may comprise reproductions or, even, translations of other work or articles previously published in other academic periodicals or scientific journals.

 

Instructions for the presentation of manuscripts

All papers submitted for publication should be sent to Salud Pública de México, Av. Universidad 655, colonia Santa María Ahuacatitlán, 62508 Cuernavaca, Morelos, México. E-mail address: eoropeza@insp3.insp.mx .

It is essential that all work should be accompanied by a cover letter stating the following:

1. That the authors agree with the contents of the paper (including tables and figures) and state the order of authorship (which will be considered definitive with no exception).
2. That authors transfer to Salud Pública de México copyright upon acceptance of publication.
3. That the paper is an original work that has not been published, neither partially nor totally, and that it has not been submitted for publication by the authors or anyone anybody to other national or foreign journals or media, in printed or electronic form.
4. That the letter is signed by each author.

Moreover, the letter must specify the author responsible for correspondence and reprint requests, as well as information for communications (complete addresses, telephone and fax numbers, and e-mail address).

Manuscripts should be submitted with two additional copies (including tables and figures); they should be typewritten, double-spaced throughout, on one side of 21 x 28 cm white paper. The approximate content of a page is 25 lines and 65 characters per line. In the event a new version is requested or the work is accepted, the document must be delivered in diskette (3.5") with the file in Word, Word Perfect or ASCII (Macintosh or IBM compatible).

Manuscripts, except brief communications and essays, must not be shorter that 10 pages of text or longer than 18 printed pages (with a maximum of 4 000 words of text), from the initial page up to and including references, with tables and figures considered separately.

Authors must keep a copy of all material sent, as manuscripts submitted for review are not returned to authors, regardless of whether or not they are published.

The manuscript’s components should observe the following structure:

· Front page
· Abstracts and key words (in English and Spanish)
· Text
· Acknowledgments
· References
· Tables and figures

A. Front page

The front page must contain the work’s complete title, written in a clear and precise manner, as well as the suggestion for a short title not exceeding five words. Next, a list with the full names of the authors, indicating the order in which they are to appear in the publication; it is important for this order to be definitive, as it cannot be modified subsequently. Each author’s academic degrees must be specified, as well as his or her institutional affiliation. Salud Pública de México does not accept original articles with more than six authors (unless they are multi-center studies, in which case the authors may number as many as eight), nor does it accept essays and review articles with more than three authors. In the latter case, more authors may be accepted when the work is accompanied by a justification that clarifies with absolute certainty the contribution of each one of the authors in the completion of the work submitted and not necessarily in the research project. Even when the number of authors adheres to these guidelines, such justification may be requested when the Editorial Board deems it appropriate. In this regard it is worth noting that these guidelines follow the requirements of the International Committee of Medical Journal Editors,5 which set forth:

Authorship will be credited solely on the basis of the writer’s essential contribution as to: a) conception and design of the study, or analysis and interpretation of the data; b) the writing of the article or the critical revision of a part of its intellectual content; and c) final approval of the version to be published. Requirements a, b and c must always be fulfilled. Participation that consists merely in obtaining financing or collecting data does not justify granting anyone authorship. Neither does general supervision of the group of researchers suffice. Any part of the article that is essential to the main conclusions should be the responsibility of at least one of the authors. 

In an article by a corporate (collective) author, those persons who are chiefly responsible for the document must be specified; all other individuals who have collaborated in the work will be acknowledged in a separate section (see "Acknowledgments"). 

The journal’s editors may request the authors to justify the attribution of authorship. 

It is important to point out that when information generated by other institutions is used, either independently or jointly, authors must request the necessary permission and give credit to the corresponding source.

B. Abstract in Spanish and key words

On the second page of the manuscript, authors must present an abstract in Spanish; in the case of original and brief communications, the abstract should be structured so as to indicate specifically: objective, materials and methods, results and major conclusions. Authors must also provide from three to six key words to make possible rapid identification of the article’s subject-matter; for choice of key words, we recommend that authors turn to the controlled vocabulary used by the following indexes of descriptors: Medical Subject Headings, of Index Medicus; Descriptores en Ciencias de la Salud, of the Red BIREME -Centro Latinoamericano y del Caribe de Información en Ciencias de la Salud, and Bibliomex Salud, of the Universidad Nacional Autónoma de México/Secretaría de Salud/Instituto Mexicano del Seguro Social. Abstracts should be limited to 150 words in the case of original articles, and to 100 words for brief communications.

C. Title, abstract and key words in English

The third page corresponds to the translation into English of the title of the article and of the abstract. If necessary, it may be longer than the original abstract, for it is important that it contains sufficient information so that the English-language reader is able to learn about the article’s basic data. The authors should also include translation of the key words. The suggested length is 150 and 100 words for original articles and brief communications, respectively.

D. Text

This part of the article should begin on the fourth page of the manuscript. When the article deals with research reports, the text of the writing should contain the following sections:

Introduction. The author must state the purpose of the work and an account of some historical background of the study.

Materials and methods. This section will include the selection procedures of the experimental material utilized (human or animal) and will identify the methods, equipment and procedures in sufficient detail to permit their reproduction. All procedures carried out in humans should conform to the ethical norms on human experimentation announced in the Helsinki Declaration of 1975. On no account will the persons who participated in the study be identified. The drugs and chemical products used should be mentioned, including their generic names, dosage and means of administration. The mathematical procedures and statistical methods should be described in detail.

Results. Findings will be presented following a logical sequence, both in the text and in the tables and figures. Data displayed in the tables and figures should not be repeated in the text; here, authors will only comment on or summarize the most important observations. The total number of tables and/or figures should not exceed five.

Discussion. In this section authors will highlight the study’s most innovative and important aspects and will set forth the conclusions. Data presented in the section on findings should be summarized. Findings are usually compared here with those of similar studies. Author must avoid assertions and conclusions not supported by research findings.

Acknowledgments. Under the guidelines of the International Committee of Medical Journal Editors, this section includes:5

a) collaborations that should be acknowledged but that do not justify authorship, such as the general support of the department head;
b) technical help received;
c) words of thanks for financial and material support, specifying the nature of the same;
d) financial relations that may give rise to a conflict of interest.

Persons who collaborated intellectually but whose participation does not justify authorship may be cited by name, adding their function or type of collaboration; for example, "scientific adviser", "critical review of the study proposal", "collection of data", "participation in the clinical assay".

Authors must secure written authorization from the persons mentioned by name in the section on acknowledgments, as readers may infer that such persons support the data and conclusions. At all times the journal reserves the right to request such authorizations.

E. References

Bibliographical references will be presented on sheets separate from the text and should be numbered consecutively according to the sequence of their appearance in the text. The text itself will include only the corresponding Arabic reference number, printed in superscript.

References should adhere to the guidelines adopted by the U.S. National Library and by the Index Medicus.1,6,7 Titles of journals should be abbreviated according to the List of Journals Indexed in Index Medicus, published annually in the January issue of said index, or as indicated in the American National Standards Institute (ANSI).8

 

1. Journal references

Journal references should include: 

a) Family name(s) and initial(s) of given name(s), followed by a period (list all authors if six or less; if more than six, list only the first six followed by "et al"). The only punctuation to be used is commas to separate one author from another, as well as a period after the last author. If an author’s two family names are to be given, they will be joined by a hyphen.
b) Full title of the article, with capitalization of only the first letter of the initial word (and of proper names), followed by a period.
c) The journal’s abbreviation, with no punctuation of its acronym or at the end.
d) Year of publication, followed by a semi-colon.
e) Volume number (in Arabic numerals), followed by a colon.
f) Page numbers (first and final) of the article, separated by a hyphen.

Examples:

Standard form
Martínez-Gutiérrez M, Alvarez-Jiménez E. El análisis de la población en México. Salud Pública Mex 1982; 24:286-294.

Anonymous
Epidemiology for primary public health care. Int J Epidemiol 1976;5:224-225.

Corporate author
Comité Internacional de Editores de Revistas Biomédicas. Nuevos Requisitos para la publicación de manuscritos en las revistas biomédicas. Rev Med IMSS 1981;19:119-125.

 

2. Books and other reviews

a) Names and initials of all authors, followed by a period (list all authors if six or less; if more than six, only the first six should be listed followed by "et al."). The only punctuation to be used are commas to separate one author from another, as well as a period after the last author. If an author’s two family names are to be given, they will be joined by a hyphen.
b) Title of book, with capitalization of only the first letter of the initial word, followed by a period.
c) Edition, if other than the first, followed by a period.
d) City where published, followed by a colon; when more than one city is indicated as place of publication, the first city mentioned is used; the city’s name may be translated into Spanish.
e) Publisher, followed by comma.
f) Year of publication (of the last edition cited if more than one), followed by a semi-colon if the volume number is to be indicated, and by a colon if page number(s) is noted.
g) Number of volume if more than one, preceded by the word "vol.", followed by a colon.
h) Page number(s) of the particular citation; in referring to a book chapter, indicate the first and last page of the chapter, separated by a hyphen.

Examples:

Standard form
Alvarez JK, Mondragón L, Becerril A. El análisis general de la salud. 3rd edition. Madrid: Editorial Salvat, 1982;vol.6:518.

Corporate author
American Medical Association, Department of Drug. AMA drug evaluation. 3rd edition. Littleton: Publishing Sciences Groups, 1977.

Editor, compiler
Rhodes AJ, Van Rooyen CE, comp. Textbook of virology: For students and practitioners of medicine and the other health sciences. 5th edition. Baltimore: Williams & Wilkins, 1968.

Chapter of a book
Weinstein L, Swartz MN. Pathogenic properties of invading microorganisms. In: Sodeman WA, comp. Pathologic physiology: Mechanisms of disease. Philadelphia: WB Saunders, 1974:457-472.

Publication of some organization
National Center for Health Statistics. Acute conditions: Incidence and associated disability, United States, July 1968-June 1969. Rockville (MD): National Center for Health Statistics, 1972.

Table 2 contains other examples for the correct presentation of references.

 

Table 2
CORRECT PRESENTATION OF REFERENCES

Articles in journals

(1) Standard article
You CH, Lee KY, Chey RY, Menguy R. Electrogastrographic study of patients with unexplained nausea, bloating and vomiting. Gastroenterology 1980;79:311-314.

(2) Corporation
The Royal Marsden Hospital Bone-marrow Transplantation Team. Failure of syngeneic bone-marrow graft without preconditioning in post-hepatitis marrow aplasia. Lancet 1977;2:742 744.

(3) No autor
Coffee drinking and cancer of the pancreas (editorial). Br Med J 1981;283:628.

(4) Volume with supplement
Magni F, Rossoni G, Berti F BN-52021 protects guinea pig from heart anaphylaxis. Pharmacol Res Commun 1988;20 suppl 5: 75-78

(5) Issue without volume number
Baumesiter AA. Origins and control of stereotyped movements.
Monogr Am Assoc Ment Defic 1978;(3): 353-384.

(6) Issue without number and without volume number
Danoek K. Skiing in and through the history of medicine. Nord Medicinhist Arsb 1982:86-100

Books and other publications

(7) Author
Colson JH, Armour WJ. Sports injuries and their treatment. 2a. edición. London, 1986.

(8) Editor(s) and compiler(s) as author(s)
Diener HC, Wilkinson M, ed. Drug induced headache. New York: Springer-Verlag, 1988.

(9) Corporation
Virginia Law Foundation. The medical and legal implications of AIDS. Charlottesville: Virginia Law Foundation, 1987.

(10) Chapter in book
Viter; F The consequences of iron deficiency and anemia in pregnancy En: Allen L, King J, Lönnerdal B, ed. Nutrient regulation during pregnancy, lactation and infant growth. Advances in experimental medicine and biology. New York: Plenum Press, 1994: 127-139. 

(11) Report presented at symposium
Harley NH. Comparing radon daughter dosimetric and risk models. En: Gammage RB, Kaye SV, ed. Indoor air and human health. Proceedings of the Seventh Life Sciences Symposium; 1984 oct 29-32; Knoxville (TN), EUA. 

(12) Thesis
Youssef NM. School adjustment of children with congenital heart disease (tesis). Pittsburgh (PA): University of Pittsburgh, 1988.

Other publications

(13) Newspaper article
Rensberg B, Specter B. CFCs may be destroyed by natural process.
The Washington Post 1989 agosto 7; sección A: 2 (col 5).

(14) Dictionary or similar references
Ectasia. Dorland's illustrated medical dictionary. 27a. edición.
Philadelphia: WB Saunders, 1988:527.

Unpublished material

(15) In press
Lillywhite HB, Donald JA. Pulmonary blood flow regulation in an aquatic snake. Science. In press.

 

F. Tables and figures

Information to be given in tables should be presented double-spaced throughout in separate sheets of paper. Tables should be numbered in Arabic numerals, in the same order in which they are mentioned in the text.

All tables should be self-explanatory, should be given a full title, although brief and concise; each column should carry headings at the top; notes and description of non-authorized abbreviations should be placed at the end of the whole table, including clear indications of the measurements used. The use of vertical and horizontal lines should be omitted.

Figures should be numbered using Arabic numerals, in the same order in which they are mentioned in the text. Photographic reproductions should be high quality, clear, contrasted in black and white and glossy printed. On the back of each photograph, one should attach a label with the corresponding Arabic number, the title of the article, the name of the principal author, and an indication noting the illustration’s right side up.

Each figure should be presented in a separate sheet, numbered sequentially, and should contain the title and the explanations of the symbols, arrows, numbers or letters used as keys for identifying the different parts of the illustration. Both tables and figures should include source notes.

 

Units of measure-International System of Units

Units of weight and measure and physical units cited in the works must correspond to the International System of Units (IS) approved by the International Office of Weights and Measurements and recommended for the health professions in WHA resolution 30.39, adopted by the Thirtieth World Health Assembly held in May of 1977.

Several rules and specifications exist regarding the use of the IS symbology:9,10

· For symbols or abbreviations, lowercase is recommended; some exceptions are: the K of the Kelvin unit of temperature and the A of the unit of electric current, the ampere; in the case of the liter the symbol can be both l or L; this unit is not registered in the IS, but frequently used in health manuscripts.
· No symbol should be followed by a period (.), unless the symbol is at the end of a sentence.
· No symbol should be expressed in the plural, all symbols will be in the singular (kg, m, mg).
· When numbers are used, they must be followed by their symbol set apart by a space between each other (100 m, 3 mol, 50 ml).

 

Publication of the accepted article

All accepted manuscripts are subject to copy editing; the editor reserves the right to make formal changes he considers appropriate to achieve greater uniformity and clarity in the presentation of the work.

Upon publication of approved papers, 20 reprints are mailed free of charge to the corresponding author.

 

Acknowledgments

The original authorship of this work was headed by Octavio Gómez Dantés, M.D., and appeared for the first time in 1988 in this journal. Since 1992, valuable contributions, made by Lucila Pacheco Peña, M.D. -who was the general editor of Salud Pública de México until 1995- have been incorporated. They have our highest recognition for their important professional work in this journal.

 

REFERENCES

1. International Steering Committee. Uniform requirements for manuscripts submitted to biomedical journals. Ann Intern Med 1979;90:95-99.         

2. International Committee of Medical Journal Editors. Uniform requirements for manuscripts submitted to biomedical journals. Ann Intern Med 1988;108:258-265.         

3. International Committee of Medical Journal Editors. Uniform requirements for manuscripts submitted to biomedical journals. Br Med J 1991;302:338-341.         

4. Frenk J. La nueva salud pública. En: Instituto Nacional de Salud Pública 1987-1991. Cuernavaca, México: INSP, 1992.         

5. International Committee of Medical Journal Editors. Uniform requirements for manuscripts submitted to biomedical journals. Can Med Assoc J 1997;156:270-277.         

6. Huth E. New form for references. Br Med J 1979;1: 1697-1698.         

7. Huth E. Medical style & format. An international manual for authors, editors, and publishers. Filadelfia, FA: ISI Press, 1987.         

8. American National Standards for Information Sciences. Abbreviation of titles of publications. Nueva York: ANSI, 1986.         

9. Velázquez-Jones L, Llópiz-Avilés M. El Sistema Internacional de Unidades en la investigación y la práctica clínica. Bol Med Hosp Infant Mex 1987;44:369-372.         

10. Llópiz-Avilés M, Gómez-Dantés O. El Sistema Internacional de Unidades. Salud Pública Mex 1988;30:905-908.         

 

 

* This work corresponds to the translation of the article: Oropeza-Abúndez C, Atrián-Salazar ML, Fuentes-Ramírez MR. Normas para la publicación de manuscritos en Salud Pública de México. Salud Pública Mex 1997;39:75-82. This document is published every year in Spanish and the present version contains substantial changes that apply from this volume onward. In addition, the article has been revised according to the most recent version of: International Committee of Medical Journal Editors. Uniform requirements for manuscripts submitted to biomedical journals. Can Med Assoc J 1997;156:270-277.

Instituto Nacional de Salud Pública Cuernavaca - Morelos - Mexico
E-mail: spm@insp3.insp.mx