STATISTICS AND RESEARCH DESIGN
Introduction to observational studies: Part 1 Nikolaos Pandis, Associate Editor of Statistics and Research Design Bern, Switzerland, and Corfu, Greece
A
s has been highlighted in previous articles of this column, randomized controlled trials (RCTs) command the highest level in the hierarchy of evidence among clinical studies. However, in some situations, RCTs are neither feasible nor ethical. A classic example from the medical field is the assessment of the association between smoking and lung-related diseases. It is easy to understand that it is not ethical to randomize patients into smoking and nonsmoking groups, and follow them to see whether one group is more likely to develop lung-related diseases in the future. An orthodontic example might be to randomize patients needing orthodontic care to treatment and nontreatment groups to assess long-term changes in periodontal status between subjects undergoing orthodontic therapy vs no orthodontic therapy. In this scenario, it would not be ethical to deprive needy patients of therapy. Another example would be to assess the effect of water fluoridation vs nonfluoridation. In this case, it would not be possible to randomize by place of residence (village, town, or city); comparisons would be feasible with participants from areas with and without water fluoridation and, therefore, without random allocation. In those situations, we observe patients and collect data without making decisions about their exposure and the course of events. Those studies are called observational; the Figure shows how they relate to interventional studies (clinical trials). Observational studies can be prospective, retrospective, longitudinal, cross-sectional, descriptive, or analytic; those characteristics are shown in the Table. In prospective studies, information on exposure is collected from the start of the study, and information on the outcome is collected in the future. For example, patients undergoing orthodontic therapy and patients not having orthodontic therapy can be recruited, and information on some outcome—eg, periodontal status— can be collected in the future. Comparisons between treated and untreated subjects in terms of their periodontal status are made between those exposed and
Am J Orthod Dentofacial Orthop 2014;145:119-20 0889-5406/$36.00 Copyright Ó 2014 by the American Association of Orthodontists. http://dx.doi.org/10.1016/j.ajodo.2013.09.005
Fig. Main categories of clinical studies.
Table. Main categories of clinical studies Prospective
Information on exposure is collected at the beginning of the study and information on the outcome in the future
Retrospective
Information on exposure is collected retrospectively The interest is to see the route of events over time
Longitudinal
Cross-sectional Information on the exposure and outcome is collected at the same time (snapshot) Descriptive Distribution of an outcome at a particular time point is described Analytical
Associations between risk factors and outcome of interest are explored
unexposed to orthodontic therapy. Again, it would be unethical to randomize patients needing orthodontic therapy to treatment or no treatment, but it would be acceptable to follow patients choosing to receive or not to receive orthodontic therapy. Retrospective studies are those in which information is collected from existing records or possibly from participants’ interviews; in those studies, in general, we are looking back. To use the previous example, we can currently assess the periodontal status of a large group of patients and then interview them or retrieve information from existing records and determine who had 119
Statistics and research design
120
received orthodontic therapy in the past and who did not. By recording current periodontal status and previous exposure to orthodontic therapy, we can determine whether there is an association between orthodontic therapy and periodontal status. The question that we are interested in answering would be “Is there a difference in periodontal status between patients who had orthodontic therapy compared with those who did not?” If more patients with a compromised periodontal status had undergone orthodontic therapy, we might start thinking that there is a potential association. However, we should be careful not to draw quick conclusions from observational studies; we will explore this in more detail in future articles. In longitudinal studies, we are interested in looking at study participants over time, and the previous examples of prospective and retrospective studies might fall under the umbrella of longitudinal studies, since either the patients were followed for a number of years or information on some exposure from the past was associated with an outcome in the future. Cross-sectional studies collect information on the exposure and the outcome at the same time. Staying with the same theme, we can collect information on
January 2014 Vol 145 Issue 1
the prevalence of periodontitis in men and women in the 20- to 30-year age group (descriptive). Periodontal status information can be collected, and an assessment of whether an association exists between sex and periodontitis can be made (analytical). Finally, descriptive studies are those that collect information in a particular time point on the prevalence, for example, of patients undergoing orthodontic treatment in an area. In analytical studies, associations can be explored between risk factors and outcomes, such as an association between maxillary incisor agenesis and maxillary canine impaction. There are 4 major types of observational studies, and we will discuss all of them except the last category in a separate article (Fig). KEY POINTS
1. 2. 3.
Observational studies are often used when RCTs are not possible or ethical. Observational studies are more prone to bias and confounding compared with RCTs. Observational studies should be interpreted with caution.
American Journal of Orthodontics and Dentofacial Orthopedics
Introduction to observational studies: Part 1 Nikolaos Pandis, Associate Editor of Statistics and Research Design Bern, Switzerland, and Corfu, Greece
A
s has been highlighted in previous articles of this column, randomized controlled trials (RCTs) command the highest level in the hierarchy of evidence among clinical studies. However, in some situations, RCTs are neither feasible nor ethical. A classic example from the medical field is the assessment of the association between smoking and lung-related diseases. It is easy to understand that it is not ethical to randomize patients into smoking and nonsmoking groups, and follow them to see whether one group is more likely to develop lung-related diseases in the future. An orthodontic example might be to randomize patients needing orthodontic care to treatment and nontreatment groups to assess long-term changes in periodontal status between subjects undergoing orthodontic therapy vs no orthodontic therapy. In this scenario, it would not be ethical to deprive needy patients of therapy. Another example would be to assess the effect of water fluoridation vs nonfluoridation. In this case, it would not be possible to randomize by place of residence (village, town, or city); comparisons would be feasible with participants from areas with and without water fluoridation and, therefore, without random allocation. In those situations, we observe patients and collect data without making decisions about their exposure and the course of events. Those studies are called observational; the Figure shows how they relate to interventional studies (clinical trials). Observational studies can be prospective, retrospective, longitudinal, cross-sectional, descriptive, or analytic; those characteristics are shown in the Table. In prospective studies, information on exposure is collected from the start of the study, and information on the outcome is collected in the future. For example, patients undergoing orthodontic therapy and patients not having orthodontic therapy can be recruited, and information on some outcome—eg, periodontal status— can be collected in the future. Comparisons between treated and untreated subjects in terms of their periodontal status are made between those exposed and
Am J Orthod Dentofacial Orthop 2014;145:119-20 0889-5406/$36.00 Copyright Ó 2014 by the American Association of Orthodontists. http://dx.doi.org/10.1016/j.ajodo.2013.09.005
Fig. Main categories of clinical studies.
Table. Main categories of clinical studies Prospective
Information on exposure is collected at the beginning of the study and information on the outcome in the future
Retrospective
Information on exposure is collected retrospectively The interest is to see the route of events over time
Longitudinal
Cross-sectional Information on the exposure and outcome is collected at the same time (snapshot) Descriptive Distribution of an outcome at a particular time point is described Analytical
Associations between risk factors and outcome of interest are explored
unexposed to orthodontic therapy. Again, it would be unethical to randomize patients needing orthodontic therapy to treatment or no treatment, but it would be acceptable to follow patients choosing to receive or not to receive orthodontic therapy. Retrospective studies are those in which information is collected from existing records or possibly from participants’ interviews; in those studies, in general, we are looking back. To use the previous example, we can currently assess the periodontal status of a large group of patients and then interview them or retrieve information from existing records and determine who had 119
Statistics and research design
120
received orthodontic therapy in the past and who did not. By recording current periodontal status and previous exposure to orthodontic therapy, we can determine whether there is an association between orthodontic therapy and periodontal status. The question that we are interested in answering would be “Is there a difference in periodontal status between patients who had orthodontic therapy compared with those who did not?” If more patients with a compromised periodontal status had undergone orthodontic therapy, we might start thinking that there is a potential association. However, we should be careful not to draw quick conclusions from observational studies; we will explore this in more detail in future articles. In longitudinal studies, we are interested in looking at study participants over time, and the previous examples of prospective and retrospective studies might fall under the umbrella of longitudinal studies, since either the patients were followed for a number of years or information on some exposure from the past was associated with an outcome in the future. Cross-sectional studies collect information on the exposure and the outcome at the same time. Staying with the same theme, we can collect information on
January 2014 Vol 145 Issue 1
the prevalence of periodontitis in men and women in the 20- to 30-year age group (descriptive). Periodontal status information can be collected, and an assessment of whether an association exists between sex and periodontitis can be made (analytical). Finally, descriptive studies are those that collect information in a particular time point on the prevalence, for example, of patients undergoing orthodontic treatment in an area. In analytical studies, associations can be explored between risk factors and outcomes, such as an association between maxillary incisor agenesis and maxillary canine impaction. There are 4 major types of observational studies, and we will discuss all of them except the last category in a separate article (Fig). KEY POINTS
1. 2. 3.
Observational studies are often used when RCTs are not possible or ethical. Observational studies are more prone to bias and confounding compared with RCTs. Observational studies should be interpreted with caution.
American Journal of Orthodontics and Dentofacial Orthopedics
