[Downloaded free from http://www.thejhs.org on Tuesday, August 09, 2016, IP: 41.128.165.40] Original Article Selecting the appropriate study design for your research: Descriptive study designs Aamir Omair Department of Medical Education, Research Unit, King Saud bin Abdulaziz University for Health Sciences, Riyadh, Saudi Arabia ABSTRACT This article describes the importance of selecting the appropriate epidemiological study design for a given study question. It provides an explanation to the different terms used in describing study designs with regards to observational versus interventional and descriptive versus analytical types of study designs. This article focuses on the description of the different types of descriptive study designs, that is, case report, case series, correlational, and cross-sectional study designs. The requirements for selecting these study designs are discussed along with the advantages and disadvantages of each study design. The descriptive studies are similar in the context that they are based on a single sample with no comparative group within the study design. Their basic purpose is to describe the characteristics of the sample with regards to the characteristics that are present and so are useful in generating a hypothesis. The absence of a comparative group is the main limitation of the descriptive studies, and this is the reason they cannot be used to determine an association by testing a hypothesis showing a relationship between a risk factor and disease. The analytical study designs will be discussed in the next article in this series. Keywords: Case report, case series, correlational study, cross-sectional study INTRODUCTION research and the next article in this series will focus on analytical study designs. One of the basic issues in deciding how to conduct a study is to first determine the appropriate epidemiological The quantitative research study designs are broadly study design for achieving the stated aim and objectives classified either as descriptive versus analytical study of the proposed research question.[1] Choosing the designs or as observational versus interventional. Study right study design is the most important decision to designs are arranged in a hierarchy starting from the make in determining the methodology of any research basic ‘Case Report’ to the highly valued ‘Randomised study. This is important for the way in which the Clinical Trial’[4] as shown in Table 1. Before discussing study will be conducted, especially the sampling the advantages and disadvantages of each study design, and data analysis.[2,3] Study designs are different for it is important to understand what is meant by the qualitative and quantitative research. The qualitative terms ‘descriptive’, ‘analytical’, ‘observational’, and study designs will be discussed separately in an article ‘interventional’, as used to classify the different types on qualitative research. This article will describe the of study. types of descriptive study designs used in quantitative Access this article online CLASSIFICATION OF STUDY DESIGNS Quick Response Code: Website: Descriptive study designs are useful for simply www.thejhs.org describing the desired characteristics of the sample that is being studied, e.g., an abnormal presentation of a disease in a case report or a case series which includes DOI: a collection of cases with the same disease/condition. 10.4103/1658-600X.159892 A descriptive study may also try to generalise the findings from a representative sample to a larger target Address for correspondence: population as in a cross-sectional survey.[5] The common Dr. Aamir Omair, Department of Medical Education, Research Unit, aspect between the descriptive study designs is that King Saud bin Abdulaziz University for Health Sciences, there is only one single sample without any comparison Riyadh, Saudi Arabia. group.[6] Analytical study designs, on the other hand, E-mail: [email protected] start with the presumption of comparing two or more Journal of Health Specialties / July 2015 / Vol 3 | Issue 3 153 [Downloaded free from http://www.thejhs.org on Tuesday, August 09, 2016, IP: 41.128.165.40] Omair: Descriptive study designs groups and follows them over a period of time. The term Table 1: The hierarchy of epidemiological study designs retrospective refers to a collection of data pertaining to Observational studies Strength of events which have occurred in the past, irrespective of evidence how/when the subjects are enrolled in the study. Hence, Descriptive study designs it is more appropriate to reserve the terms prospective [9] Case report Single case and retrospective only for cohort studies. Case series Collection of similar cases Correlational Population based study - using The choice of the appropriate study design depends secondary data upon the way the research question is stated.[1] It is Cross-sectional Single sample from larger important to note here that different study designs (descriptive) population- no comparison may be applicable for the same research problem, but Analytical study designs it is the way that the research question is framed that Cross-sectional Single sample from larger determines which study design is most appropriate. It (analytical) population-compares two or more groups in the sample should also be added that there are many grey areas in Case-control Compares risk factors which researchers may differ in opinion about the type between diseased (cases) and of study design, but the important factors to consider non-diseased (controls) groups include the research question as well as the way the Cohort Compares outcomes between subjects were selected for the study.[3,6] We will consider groups exposed and non-exposed to a risk factor for a disease ‘shisha smoking and heart disease’ as an example for Interventional study identifying the application of the different types of Clinical trial Investigator allots the subjects study design in this article. The research problem is to different groups - intervention that the practice of shisha (hubble-bubble) smoking is versus non-intervention becoming very common and is not considered by the general public as a serious health problem as compared groups and the samples are selected accordingly from to cigarette smoking. the different groups. These groups may be based on people who are diseased/not diseased for a ‘case-control CASE REPORT/CASE SERIES study’, or they may be based on people who are exposed/ A case report in this example may just present a study not exposed to a particular risk factor in a ‘cohort of a young adult aged less than 30 years who has study’.[3] A clinical trial is the third type of analytical been smoking shisha since the age of 8 years with his study design in which one group is considered as elder brothers. The case report is a presentation of an the intervention (or experimental) group, which is abnormal finding or outcome which, otherwise, would compared to the non-intervention group (comparison) not be present. It describes the case in detail presenting [7] group. the abnormal findings but is generally limited in demonstrating an association between the risk factor A clinical trial is classified as an ‘interventional’ and the disease.[10] The argument is that this finding study because the investigator determines who is may be due to chance or coincidence, without there to be placed in the experimental or the comparison being a real association between the risk factor and group. All the other types of study designs are the outcome. Another example may be of an adverse classified as ‘observational’ since the investigator (or beneficial) side-effect of a new drug, which may only labels a subject as being diseased/non-diseased not have been documented before. These case reports or exposed/non-exposed based on his/her previous are useful in identifying a new phenomenon and the status.[8] The terms prospective and retrospective, as reporting of similar cases from other observers, thereby applicable to epidemiological study designs, refer to helping generate a hypothesis for the association whether a subject is being followed up in the future or between exposure and outcome.[10] are being asked/investigated about events or exposure in the past.[8] These terms are now used only for the A “Case Series” is a collection of cases with the same cohort studies among the different observational outcome/finding. The general number of cases reported study designs. The collection of subjects or data in the in a case series range from 20 to 50, but may vary from future does not classify a study as ‘prospective’ so it is as few as 5 to as many as more than 100. In the above not appropriate to classify a cross-sectional study as example, a study may show a number of young adult a prospective study even though the subjects may be ‘shisha smokers’ presenting with heart disease. These enrolled over a period of time. An interventional study kind of studies are more useful than case reports design such as a clinical trial is generally prospective in generating a viable hypothesis for an association since the investigator assigns the subjects to different between exposure and outcome.[11] However, there is 154 Journal of Health Specialties / July 2015 / Vol 3 | Issue 3 [Downloaded free from http://www.thejhs.org on Tuesday, August 09, 2016, IP: 41.128.165.40] Omair: Descriptive study designs still a chance of this being only an incidental finding Correlational studies have the advantage of being quick and the real reason may be due to some ‘other’ common and inexpensive since they are based on secondary factor. For example in the above study, there may be data, which is already available from different sources. other risk factors such as cigarette smoking, lack of However, it should be kept in mind that any probable exercise, unhealthy diet and family history, etc. The association shown in the correlational studies might main limitation of a case series study is the absence of be due to some other underlying factor and not due a comparative group.