Types of Epidemiological Studies in Peruvian Biomedical Journals M any decisions made in the daily practice of med- under the headings above(editorials, clinical or icine are based on advances in scientific knowledge, pathological discussions, interviews, book journals being one of the more important means for reviews, translations of foreign articles, etc.). its dissemination. For this reason, journals must provide reliable and truthful information concern- The number of articles for each journal was the ing advances in science. sum of those given in the index. The articles consi- Recently there has been special interest in study- dered to be epidemiological were classified in turn ing the nature, reliability, and reproducibility of according to the type of epidemiological study " -;", information offered in specialized journals. Thus, the type of statistical analysis used "", and the nature for example, Feinstein "', Fletcher and Fletcher ', of the references cited. and Bailar '-, among others, have designed classifi- The types of epidemiological studies considered cation schemes for biomedical research which make were: it possible to do a critical analysis of studies, to identify the most powerful methods of research, and 1. Descriptive to systematize it. On the other hand, several authors ,4-" have found the frequency with which important - Strictly descriptive: details or characteristics data are omitted related to the design and presenta- of a phenomenon or situation are presented tion of results is very high, even in serious journals without any intention of establishing associ- of worldwide circulation. ations such as case reporting, surveys, stu- In the present study an evaluation has been made dies on incidence, prevalence, mortality, of information contained in the regularly published morbidity (209 studies, 71%). Peruvian medical journals of widest circulation, Descriptive of effectiveness: observations on focusing on the type of study published, with the effect of a treatment or procedure in a emphasis on those of epidemiological content, and group of persons without any control group, reviewing the statistical analyses used and the bibli- such as uncontrolled trials (65 studies, 22%). ography cited. The study was initiated and deve- loped almost totally as an undergraduate curricu- 2. Analytical: Studies that attempt to establish an lum experiment in the Comprehensive Health association or causal relation, or to evaluate pro- Course at the Alberto Hurtado School of Medicine, posed hypotheses. Universidad Peruana Cayetano Heredia, Peru. Reviews were undertaken of all the articles - Retrospective: a study of cases and controls printed in the journals Tribuna Médica, Diagnós- in which the primary observation is the tico, Galeno, and Revista de Neuropsiquiatría pub- effect, based on which the imputed factor is lished between 1979 and 1983, plus issues (6) 1-3, (7) traced as the cause (one study). I and 2, (9) 1 and 2, and (1 1) 2 of the journal Acta Cross-sectional: a study that analyzes the Médica Peruana. Each article was classified accord- association between two apparently inde- ing to the type of study, its motivation and the style pendent characteristics or situations ob- of presentation, according to the following hier- served simultaneously, for which the direc- archy: tion of the cause-effect relation is imprecise (9 studies). 1. Epidemiological: original study of manifesta- - Prospective: a study based on the longitudi- tions, therapeutic tests, causality, diseases or nal follow-up of a particular cohort which attributes in groups or populations. attempts to examine the effect of exposure to 2. Technological: study whose principal objective a factor imputed to be causal without any is to demonstrate the validity or merits of a manipulation by the observer (2 studies). technique or procedure (sonography, tomo- Experimental: evaluation of a treatment graphy, etc.). through the comparison of a group exposed 3. Review: article that selects, synthesizes, or to the treatment and a comparable control evaluates information already published, with group selected randomly based on a double- or without examples of clinical cases. blind design (randomized controlled trials) 4. M inor publications: other articles not included (3 studies). 6 - Pre-experimental: a study defined in the standardize the definitions, confirm the type of same terms as the previous type except for epidemiological study, and discuss any mistakes the double-blind design and the random made in the design of the studies. In general there selection of the subjects, such as the non- was no difficulty in reaching agreement. random controlled trials (2 studies). R esults 3. Methodological: Studies of sensitivity and/or specificity (4 studies). Table I shows the overall breakdown of the arti- cles according to the type of study. It is observed The articles were assigned to one of these catego- that most of them are review articles (43%), fol- ries depending on the study design and on the results lowed by minor publications (38%), epidemiologi- obtained, independently of the authors' opinion '' cal studies (15%), and finally, technological articles According to the statistical methods utilized, the (5%). It was found, in addition, that Acta Médica studies were subdivided as: Peruana and Galeno were the journals with the largest percentages of epidemiological studies, and 1. No statistical method or descriptive statistics Diagnóstico and Tribuna Médica, of review arti- only: percentages, means, standard devia- cles. tion, histogram, etc. (257 studies, 83%). Of the epidemiological studies, 93% are descrip- 2. One-sample t test, paired t test, two samples tive, 5% analytical, and 1% methodological. This test, z transformation test (16 sttidies, 5%). same trend was observed for each of the journals 3. Contingency tables, Chi squared test, individually, with the exception of Acta Médica Fisher's ideal index, McNemar's test (7 Peruana (100% descriptive). studies). With regard to the type of statistical analysis 4. Other non-parametric tests of the sign, employed, the great majority of epidemiological Mann-Whitney, Wilcoxon (one study). articles (257) used only descriptive statistics and the 5. Epidemiological statistics: relative risk, sensi- 37 remainder ones used a total of 51 statistical tivity, specificity, rates (five studies). methods. The method most used was the t test, 6. Simple linear regression, correlation coeffi- followed by contingency tables. cient (5 studies). Table 1. Breakdown of articles by type of study. 7. Life tables: actuarial, Kaplan-Meier estimate of survival (2 studies). Articles 8. Survival regression: Cox regression and Type of study No. % logistic regression. 9. Analysis of variance, covariance and F test. Epidemiological 294 14.8 10. Multiple regression: polynomial, step-wise. Non-epidemiological 11. Logarithmic transformation to be used in - Technological 91 4.6 another test. - Informative review 852 42.8 - Minor publication 12. Other: any other analysis not mentioned (editorials. interviews, above (calculus of probability, K olmogorov- díscussions) 752 37.8 Smirnov goodness-of-fit test, etc.). Total 1.989 100.0 In articles that used any other statistical method besides the descriptive one, the latter was not consi- Table 2 shows that most of the descriptive epide- dered. Articles which conclude there are significant miological studies do not use statistical analyses and differences between some of the parameters studied that most of the analytical and all the methodologi- but which did not specify the statistical method cal articles do use some type of analysis. used, were classified as descriptive statistics. The classification of articles '- "" was done by two Table 2. Statistical methods by type persons independently; in the case of discrepancies, of epidemiological study. the two reviewers analyzed the article together and No method Some method came to a decision. If not achieved, the article was No. % No. % reviewed by all the authors. Given the special struc- ture of the journal Galeno these procedures were jointly from the start. Descriptive 252 91.9 22 8.1 carried out Analytical 5 29.4 12 70.6 All the articles indicated to be epidemiological Methodological 0 0 4 100.0 were jointly reviewed by four or more authors, to 7 In table 3 it is shown that out of 287 epidemiologi- The differences found between the foreign and cal articles, 175 (61%) do not cite any reference to Peruvian journals in terms of proportions of analyt- national journals, while only 26 articles (9%) lack ical studies are notable; the preponderance of des- references to foreign journals. It is also noted that criptive studies in Peruvian publications places only 4 articles have more than 10 references to them at the most elementary level of scientific national journals. research. While in Peruvian journals only 16% of the Table 3. Characteristics of the references cited epidemiological studies utilize statistical methods in the epidemiological articles. that are not merely descriptive, in foreign journals this proportion rises to 42%, according to Emerson Articles ""', and to 47%, according to Háyden "". This gives Type of reference No. of references No. % an idea of the poverty of the statistical content of the Peruvian publications. As was to be expected, the National None 175 61.0 analytical studies use more statistical methods than 1-4 98 34.1 the descriptive ones; what is notable is that in almost 5-18 10 3.5 Over 10 4 1.4 a third of the analytical