Observational Studies 1 (2015) 124-125 Submitted 8/15; Published 8/15 Introduction to Observational Studies and the Reprint of Cochran's paper \Observational Studies" and Comments Dylan S. Small [email protected] Department of Statistics, University of Pennsylvania Philadelphia, PA 19104, USA In this first issue of Observational Studies, we reprint a review of observational studies by William Cochran, a pioneer of statistical research on observational studies, followed by comments by leading current researchers in observational studies. Cochran (1965, Journal of the Royal Statistical Society, Series A) defined an observational study as an empiric investigation [in which]...the objective is to elucidate cause-and-effect relationships...[in which] it is not feasible to use controlled experimentation, in the sense of being able to impose the procedures or treatments whose effects it is desired to discover, or to assign subjects at random to different procedures. Observational Studies is a new peer-reviewed journal that seeks to publish papers on all aspects of observational studies. Researchers from all fields that make use of observational studies are encouraged to submit papers. Topics covered by the journal include, but are not limited, to the following: • Study protocols for observational studies. The journal seeks to promote the planning and transparency of observational studies. In addition to publishing study protocols, the journal will publish comments on the study protocols and allow the authors of the study protocol to respond to the comments. • Methodologies for observational studies. This includes statistical methods for all as- pects of observational studies and methods for the conduct of observational studies such as methods for collecting data. In addition to novel methodological articles, the journal welcomes review articles on methodology relevant to observational studies as well as illustrations/explanations of methodologies that may have been developed in a more technical article in another journal. • Software for observational studies. The journal welcomes articles describing software relevant to observational studies. • Descriptions of observational study data sets. The journal welcomes descriptions of observational study data sets and how to access them. The goal of the descriptions of observational study data sets is to enable readers to form collaborations, to learn from each other and to maximize use of existing resources. The journal also encourages submission of examples of how a publicly available observational study database can be used. ⃝c 2015 Dylan S. Small. • Analyses of observational studies. The journal welcomes analyses of observational studies. The journal encourages submissions of analyses that illustrate use of sound methodology and conduct of observational studies. The paper we reprint of Cochran's and the comments by leading current researchers in observational studies provide illuminating perspectives on important issues in observational studies that the journal seeks to address. The contents of the rest of this section are as follows: Author Title Pages William Cochran Observational Studies 126-136 Norman Breslow William G. Cochran and the 1964 Surgeon's 137-140 General Report Thomas Cook The Inheritance bequeathed to William G. 140-163 Cochran that he willed forward and left for others to will forward again: The Limits of Observational Studies that seek to Mimic Randomized Experiments David Cox & Nanny Wermuth Design and interpretation of studies: relevant 165{170 concepts from the past and some extensions Stephen Fienberg Comment on \Observational Studies" 171{172 by William G. Cochran Joseph Gastwirth & Barry Graubard Comment on Cochran's \Observational Studies 173{181 Andrew Gelman The State of the Art in Causal Inference: 182{183 Some Changes Since 1972 Ben Hansen & Adam Sales Comment on Cochran's \Observational Studies" 184{193 Miguel Hern´an A good deal of humility: 194{195 Cochran on observational studies Jennifer Hill Lessons we are still learning 196{199 Judea Pearl Causal Thinking in the Twilight Zone 200{204 Paul Rosenbaum Cochran's Causal Crossword 205{211 Donald Rubin Comment on Cochran's \Observational Studies" 212{216 Herbert Smith Comment on Cochran's \Observational Studies" 217{219 Mark van der Laan Comment on \Observational Studies" 220{222 by Dr. W.G. Cochran (1972) Tyler VanderWeele Observational Studies and Study Designs: 223{230 An Epidemiologic Perspective Stephen West Reflections on \Observational Studies": 231{240 Looking Backward and Looking Forward 125 Observational Studies 1 (2015) 126-136 Submitted 1972; Published reprinted, 8/15 Observational Studies William G. Cochran Editor's Note: William G. Cochran (1909-1980) was Professor of Statistics, Harvard University, Cambridge, Massachusetts. This article was originally published in Statistical Papers in Honor of George W. Snedecor, ed. T.A. Bancroft, 1972, Iowa State University Press, pp. 77-90. The paper is reprinted with permission of the copyright holder, Iowa State University Press. Comments by leading current researchers in observational studies follow. 1. Introduction OBSERVATIONAL STUDIES are a class of statistical studies that have increased in fre- quency and importance during the past 20 years. In an observational study the investigator is restricted to taking selected observations or measurements on the process under study. For one reason or another he cannot interfere in the process in the way that one does in a controlled laboratory type of experiment. Observational studies fall roughly into two broad types. The first is often given the name of \analytical surveys." The investigator takes a sample survey of a population of interest and proceeds to conduct statistical analyses of the relations between variables of interest to him. An early example was Kinsey's study (1948) of the relation between the frequencies of certain types of sexual behavior and variables like the age, sex, social level, religious affiliation, rural-urban background, and direction of social mobility of the person involved. Dr. Kinsey gave much thought to the methodological problems that he would face in planning his study. More recently, in what is called the \midtown Manhattan study" (Srole et al., 1962), a team of psychiatrists studied the relation in Manhattan, New York, between age, sex, parental and own social level, ethnic origin, generation in the United States, and religion and nonhospitalized mental illness. The second type of observational study is narrower in scope. The investigator has in mind some agents, procedures, or experiences that may produce certain causal effects (good or bad) on people. These agents are like those the statistician would call treatments in a controlled experiment, except that a controlled experiment is not feasible. Examples of this type abound. A simple one structurally is a Cornell study of the effect of wearing a lap seat belt on the amount and type of injury sustained in an automobile collision. This study was done from police and medical records of injuries in automobile accidents. The prospective smoking and health studies (1964) are also a well-known example. These are comparisons of the death rates and causes of death of men and women with different smoking patterns in regard to type and amount. An example known as the \national halothane study" (Bunker et al., 1969) attempted to make a fair comparison of the death rates due to the five leading anesthetics used in hospital operations. ⃝c 2015 Iowa State University Press. Observational Studies Several factors are probably responsible for the growth in the number of studies of this kind. One is a general increase in funds for research in the social sciences and medicine. A related reason is the growing awareness of social problems. A study known as the \Coleman report" (1966) has attracted much discussion. This was begun because Congress gave the U.S. Office of Education a substantial sum and asked it to conduct a nation wide survey of elementary schools and high schools to discover to what extent minority-group children in the United States (Blacks, Indians, Puerto Ricans, Mexican-Americans, and Orientals) receive a poorer education than the majority whites. A third reason is the growing area of program evaluation. All over the world, administrative bodies { central, regional, and local { spend the taxpayers' money on new programs intended to benefit some or all of the population or to combat social evils. Similarly, a business organization may institute changes in its operations in the hope of improving the running of the business. The idea is spreading that it might be wise to devote some resources to trying to measure both the intended and the unintended effects of these programs. Such evaluations are difficult to do well, and they make much use of observational studies. Finally, some studies are undertaken to investigate stray reports of unexpected effects that appear from time to time. The halothane study is an example; others are studies of side effects of the contraceptive pill and studies of health effects of air pollution. This paper is confined mainly to the second, narrower class of observational studies, although some of the problems to be considered are also met in the broader analytical ones. For this paper I naturally sought a topic that would reflect the outlook and research interests of George Snedecor. In his career activity of helping investigators, he developed a strong interest in the design of experiments, a subject on