Global Disability Research: Types of Study

In this article, we will consider some of the typical study designs used in public health research, with particular focus on those that are commonly used in the field of disability.

The first step in a research cycle is to develop a research question. After you have developed a question, the study design must be considered. But how do you choose what type of study to do? There are a range of study designs, but they can be divided into a few broad categories. The first distinction to make is whether the study is observational, experimental or a summary.

Observational studies

Observational studies are non-experimental in nature, and are used to observe a phenomenon of interest without imposing any changes (e.g. providing interventions). These types of studies are used to address questions about prevalence (e.g. the proportion (%) of individuals living with disability in a given population), causes, risk groups, or lived experiences (e.g. access to water and sanitation for people with disabilities). Observational studies include cross-sectional, cohort, and case control studies.

Cross sectional study

In a cross-sectional study, people with a disease, risk factor, or other condition of interest are identified at a single point in time. A particular type of cross-sectional study used in disability research is a population-based survey. This type of study gives us an estimate of the prevalence of the factor of interest at the time of the study. For example, we might be interested to understand the prevalence of disability in a particular country. You will hear more about population-based surveys in Week 2. Given that cross sectional studies are conducted at a single point in time, they are unable to allow us to determine whether a particular exposure or risk factor causes a condition or not (causality).

Case control study

In case control studies, people are invited to participate based on whether they have a particular condition or outcome of interest. In the field of disability, “cases” are often people with disabilities, which can be defined in different ways. We will discuss measurement of disability in Week 2 of the course. In addition to cases, participants without the condition of interest are selected as controls. In case control studies outside the field of disability, researchers are often interested to understand which risk factors led to the development of a disease. Both groups of people, cases and controls, are then asked to report on different risk factors, and then a comparison between people with and without the disease is made to obtain an “odds ratio”, which gives the researcher an idea of risk factors which are associated with the disease. In the field of disability, we are often not as concerned with which risk factors lead to the disability, but instead use case control studies to understand differences between people with and without disabilities in key areas of life, such as education, livelihoods, poverty, or access to health services. Case control studies are often “embedded” within a population-based survey, these are called “nested case-control studies”. You will hear more about case-control studies, including some examples, throughout this course.

Cohort study

Cohort describes a group of people – school children, people who live in a particular city or village, or people who undertake a particular type of work. These people are disease free at the beginning of the study, and researchers follow individuals through time to monitor whether a disease is developed. These studies are also called longitudinal studies. In the field of disability research, these studies are less commonly used. However, they can be used to understand the causes of a particular impairment. This video* provides an example where researchers studied diabetic retinopathy (DR), a cause of visual impairment. The main exposure, or risk factor, is diabetes (without DR), so individuals with diabetes are the cohort, and these people are followed up until the end of the study. At the end of the study, the characteristics of people with and without the disease are compared to see whether other risk factors might contribute to the development of DR. Another example of a cohort study in disability, is to compare death/mortality rates in people with and without disabilities, an example is provided in the see also section. Cohort studies are more resource intensive, as they take much longer than other studies such as cross-sectional population-based studies.

Qualitative study

Qualitative studies allow an exploration of the variations in people’s experiences, as well as the meanings of these experiences to different people. For example, what does disability mean to you? There are several types of methods used in qualitative studies, for example interviews, or focus groups. You will hear more about qualitative methods in Step 1.13.

Experimental studies

In experimental studies, also called interventional studies, the researcher intervenes to prevent a disease, or improve a person’s life.

Randomized control trial (RCT)

The “gold standard” or most highly regarded type of experimental study is the randomized control trial (RCT). One of the key components of an RCT is the randomization process, where participants are randomly allocated to receive an intervention (intervention group), or to standard practice/care (control group). The two groups are then followed for a period of time and then compared in terms of the outcome of interest. Sometimes, it is not possible to randomize an intervention to an individual, and instead an entire group or community is the “unit of randomization”. These types of studies are called “cluster randomized control trials”. In disability, these types of studies might be used to determine the effectiveness of an intervention to improve a person’s life – in terms of access to health services or education, or alleviating poverty. These studies are often used in impact evaluations, which you will hear about in Week 2 of the course.

Evidence synthesis

There is a third type of study, which is used to summarise or synthesise studies that have already been done on a particular topic. These are called reviews, and include systematic reviews, scoping reviews and meta-analysis. The steps typically involve searching academic databases, compiling relevant studies, assessing their quality, synthesising the evidence, and interpretation. These types of studies can help us to decide which interventions are effective or suitable for a certain population, or can provide a summary of the prevalence of a condition across a region (e.g. hearing impairment in sub-Saharan Africa). You will hear more about these types of studies in Step 1.16.

Questions to consider or reflect on 

• If you wanted to understand whether children with disabilities had more or less access to education than people without disabilities, what type of study might you use?
• You want to understand the prevalence of vision impairment in a population, what type of study design is most suitable for this topic?
• Please share your experiences of research, have you been involved in any of these types of studies? Have you heard of another type of study that is not covered above?

*Video provided by Lorna Gibson, Daksha Patel, Sally Parsley – ICEH at LSHTM