More chapters in Epidemiology for the uninitiated
As discussed in the previous chapter, one of the drawbacks of using a longitudinal approach to investigate the causes of disease with low incidence is that large and lengthy studies may be required to give adequate statistical power. An alternative which avoids this difficulty is the case-control or case-referent design. In a case-control study patients who have developed a disease are identified and their past exposure to suspected aetiological factors is compared with that of controls or referents who do not have the disease. This permits estimation of odds ratios (but not of attributable risks). Allowance is made for potential confounding factors by measuring them and making appropriate adjustments in the analysis. This statistical adjustment may be rendered more efficient by matching cases and controls for exposure to confounders, either on an individual basis (for example by pairing each case with a control of the same age and sex) or in groups (for example, choosing a control group with an overall age and sex distribution similar to that of the cases). Unlike in a cohort study, however, matching does not on its own eliminate confounding. Statistical adjustment is still required.
Selection of cases
The starting point of mostcase-control studies is the identification of cases. This requires a suitable case definition (see Chapter 2). In addition, care is needed that bias does not arise from the way in which cases are selected. A study of benign prostatic hypertrophy might be misleading if cases were identified from hospital admissions and admission to hospital was influenced not only by the presence and severity of disease but also by other variables, such as social class. In general it is better to use incident rather than prevalent cases. As pointed out in chapter 2, prevalence is influenced not only by the risk of developing disease but also by factors that determine the duration of illness. Furthermore, if disease has been present for a long time then premorbid exposure to risk factors may be harder to ascertain, especially if assessment depends on people’s memories.
Selection of controls
Usually it is not too difficult to obtain a suitable source of cases, but selecting controls tends to be more problematic. Ideally, controls would satisfy two requirements. Within the constraints of any matching criteria, their exposure to risk factors and confounders should be representative of that in the population “at risk” of becoming cases – that is, people who do not have the disease under investigation, but who would be included in the study as cases if they had. Also, the exposures of controls should be measurable with similar accuracy to those of the cases. Often it proves impossible to satisfy both of these aims.
Two sources of controls are commonly used. Controls selected from the general population (for example, from general practice age-sex registers) have the advantage that their exposures are likely to be representative of those at risk of becoming cases. However, assessment of their exposure may not be comparable with that of cases, especially if the assessment is achieved by personal recall. Cases are keen to find out what caused their illness and are therefore better motivated to remember details of their past than controls with no special interest in the study question.
Measurement of exposure can be made more comparable by using patients with other diseases as controls, especially if subjects are not told the exact focus of the investigation. However, their exposures may be unrepresentative. To give an extreme example, a case-control study of bladder cancer and smoking could give quite erroneous findings if controls were taken from the chest clinic. If other patients are to be used as referents, it is safer to adopt a range of control diagnoses rather than a single disease group. In that way, if one of the control diseases happens to be related to a risk factor under study, the resultant bias is not too large.
Sometimes interpretation is helped by having two sets of controls with different possible sources of bias. For example, a link has been suggested between the phenoxy herbicides 2,4-D and 2,4,5-T and soft tissue sarcoma. Some case-control studies to test this have taken referents from the general population, whereas others have used patients with other types of cancer. Studies using controls from the general population will tend to overestimate risk because of differential recall, whereas studies using patients with other types of cancers as controls will underestimate risk if phenoxy herbicides cause cancers other than soft tissue sarcoma. The true risk might therefore be expected to lie somewhere between estimates obtained with the two different designs.
When cases and controls are both freely available then selecting equal numbers will make a study most efficient. However, the number of cases that can be studied is often limited by the rarity of the disease under investigation. In this circumstance statistical confidence can be increased by taking more than one control per case. There is, however, a law of diminishing returns, and it is usually not worth going beyond a ratio of four or five controls to one case.
Ascertainment of exposure
Many case-control studies ascertain exposure from personal recall, using either a self administered questionnaire or an interview. The validity of such information will depend in part on the subject matter. People may be able to remember quite well where they lived in the past or what jobs they did. On the other hand, long term recall of dietary habits is probably less reliable.
Sometimes exposure can be established from historical records. For example, in a study of the relation between sinusitis and subsequent risk of multiple sclerosis the medical histories of cases and controls were ascertained by searching their general practice notes. Provided that records are reasonably complete, this method will usually be more accurate than one that depends on memory.
Occasionally, long term biological markers of exposure can be exploited. In an African study to evaluate the efficiency of BCG immunisation in preventing tuberculosis, history of inoculation was established by looking for a residual scar on the upper arm. Biological markers are only useful, however, when they are not altered by the subsequent disease process. For example, serum cholesterol concentrations measured after a myocardial infarct may not accurately reflect levels before the onset of infarction.
The statistical techniques for analysing case-control studies are too complex to cover in a book of this length. Readers who wish to know more should consult more advanced texts or seek advice from a medical statistician
Cross sectional studies
A cross sectional study measures the prevalence of health outcomes or determinants of health, or both, in a population at a point in time or over a short period. Such information can be used to explore aetiology – for example, the relation between cataract and vitamin status has been examined in cross sectional surveys. However, associations must be interpreted with caution. Bias may arise because of selection into or out of the study population. A cross sectional survey of asthma in an occupational group of animal handlers would underestimate risk if the development of respiratory symptoms led people to seek alternative employment and therefore to be excluded from the study. A cross sectional design may also make it difficult to establish what is cause and what is effect. If milk drinking is associated with peptic ulcer, is that because milk causes the disease, or because ulcer sufferers drink milk to relieve their symptoms? Because of these difficulties, cross sectional studies of aetiology are best suited to diseases that produce little disability and to the presymptomatic phases of more serious disorders.
Other applications of cross sectional surveys lie in planning health care. For example, an occupational physician planning a coronary prevention programme might wish to know the prevalence of different risk factors in the workforce under his care so that he could tailor his intervention accordingly.
- Chapter 1. What is epidemiology?
- Chapter 2. Quantifying disease in populations
- Chapter 3. Comparing disease rates
- Chapter 4. Measurement error and bias
- Chapter 5. Planning and conducting a survey
- Chapter 6. Ecological studies
- Chapter 7. Longitudinal studies
- Chapter 8. Case-control and cross sectional studies
- Chapter 9. Experimental studies
- Chapter 10. Screening
- Chapter 11. Outbreaks of disease
- Chapter 12. Reading epidemiological reports
- Chapter 13. Further reading
Cross sectional studies are used to determine prevalence. They are relatively quick and easy but do not permit distinction between cause and effect. Case controlled studies compare groups retrospectively. They seek to identify possible predictors of outcome and are useful for studying rare diseases or outcomes.What is the difference between cohort case-control and cross-sectional studies? ›
Cross sectional studies are used primarily to determine the prevalence of a problem whereas cohort studies involve the study of the population that is both exposed and non-exposed to the cause of disease development agents.What is an example of a case-control study? ›
For example, investigators conducted a case-control study to determine if there is an association between colon cancer and a high fat diet. Cases were all confirmed colon cancer cases in North Carolina in 2010. Controls were a sample of North Carolina residents without colon cancer. The odds ratio was 4.0.What is a cross-sectional study in research? ›
A cross-sectional study is a type of research design in which you collect data from many different individuals at a single point in time. In cross-sectional research, you observe variables without influencing them.What is an example of cross-sectional study? ›
Another example of a cross-sectional study would be a medical study examining the prevalence of cancer amongst a defined population. The researcher can evaluate people of different ages, ethnicities, geographical locations, and social backgrounds.What type of study is a case-control study? ›
A study that compares two groups of people: those with the disease or condition under study (cases) and a very similar group of people who do not have the disease or condition (controls).Do cross-sectional studies have a control group? ›
Does a cross-sectional study have a control group? A cross-sectional study does not need to have a control group as the population studied is not selected based on exposure.What is the difference between Case Reports Case series and cross-sectional studies? ›
A case report is the description of the clinical story of a single patient. A cross-sectional study involves a group of participants on which data is collected at a single point in time to investigate the relationship between a certain exposure and an outcome.How do you plan a case-control study? ›
- Define a study population (source of cases and controls) ...
- Define and select cases. ...
- Define and select controls. ...
- Measure exposure. ...
- Estimate disease risk associated with exposure.
- Illustrative Case Study.
- Exploratory Case Study.
- Cumulative Case study.
- Critical Instance Case Study.
A case-control study is a type of observational study. It looks at 2 sets of participants. One group has the condition you are interested in (the cases) and one group does not have it (the controls). In other respects, the participants in both groups are similar.How do you collect data from a cross-sectional study? ›
Cross-sectional data can be collected by self-administered questionnaires. Using these instruments, researchers may put a survey study together with one or more questionnaires measuring the target variable(s).What is the main problem with cross-sectional research? ›
The primary limitation of cross-sectional studies is that the temporal link between the outcome and the exposure cannot be determined because both are examined at the same time. For example, in a zoo, reproduction is found to be more commonly impaired in animals with stereotypies.Why are cross-sectional studies important? ›
The benefit of a cross-sectional study design is that it allows researchers to compare many different variables at the same time. We could, for example, look at age, gender, income and educational level in relation to walking and cholesterol levels, with little or no additional cost.What questions does a cross-sectional study answer? ›
Cross-sectional study designs are useful when: Answering questions about the incidence or prevalence of a condition, belief or situation. Establishing what the norm is for a specific demographic at a specific time.Is cross-sectional study quantitative or qualitative? ›
Although the majority of cross-sectional studies is quantitative, cross-sectional designs can be also be qualitative or mixed-method in their design.How many controls are in a case-control study? ›
Consequently, if it is time-consuming or expensive to collect data on controls, the ratio of controls to cases should be no more than 4:1. However, if the data on controls is easily obtained, there is no reason to limit the number of controls.How many subjects are in a case-control study? ›
In addition, case-control studies look at a single subject or a single case, whereas longitudinal studies can be conducted on a large group of subjects. 3.Why case-control study is done? ›
A case-control study is designed to help determine if an exposure is associated with an outcome (i.e., disease or condition of interest). In theory, the case-control study can be described simply. First, identify the cases (a group known to have the outcome) and the controls (a group known to be free of the outcome).Why is it called a cohort study? ›
The term “cohort” refers to a group of people who have been included in a study by an event that is based on the definition decided by the researcher. For example, a cohort of people born in Mumbai in the year 1980. This will be called a “birth cohort.” Another example of the cohort will be people who smoke.
There are two types of cohort studies: prospective and retrospective (or historical) cohorts. Prospective studies follow a cohort into the future for a health outcome, while retrospective studies trace the cohort back in time for exposure information after the outcome has occurred.What type of sampling is cohort? ›
Participants in a prevalent cohort are recruited according to the cross-sectional sampling method, in which one ascertains risk factors (e.g., smoking or a genetic variant) and disease status at the time of enrollment, and then prospectively follows the individuals with the disease to observe the events of interest ( ...How many controls are in a case? ›
Probably the commonest approach is to just have one control per case. This is optimal if one has a sufficient number of cases. But if there are very few cases available, then it is better to increase the number of controls up to a maximum of about 4 controls per case.What is the difference between case control and case series? ›
Case series have no control at all because only patients with a certain manifestation of outcomes are sampled (e.g., individuals with a disease or deceased individuals). In contrast, all case-control designs as well as self-controlled case-control designs have a control group.How many types of case studies are there? ›
There are generally five different types of case studies, and the subjects that they address. Every case study, whether explanatory or exploratory, or intrinsic or instrumental, fits into one of these five groups.How many subjects are in a case series? ›
This journal has clear instructions to authors that a “case series” should report on no more than 10 patients. An observational study of more patients based on rates is a different category and should be possibly labeled as a rate-based descriptive study.What are the five steps to write a case study? ›
- Read and Examine the Case Thoroughly. Take notes, highlight relevant facts, underline key problems.
- Focus Your Analysis. Identify two to five key problems. ...
- Uncover Possible Solutions/Changes Needed. ...
- Select the Best Solution.
- Executive Summary. Explain what you will examine in the case study. ...
- Background. Provide background information and the most relevant facts. ...
- Case Evaluation. ...
- Proposed Solutions. ...
- Conclusion. ...
- Implementation. ...
Logistic regression modeling, in its various forms, has become by far the most frequently applied method for multivariable analysis of case-control studies.What are the 6 parts of case study? ›
- Preparation. Just like with any study, it's important to first prepare to conduct the case analysis. ...
- Introduction. ...
- Background information. ...
- Proposed solutions. ...
- Recommendations. ...
Case studies tend to focus on qualitative data using methods such as interviews, observations, and analysis of primary and secondary sources (e.g. newspaper articles, photographs, official records). Sometimes a case study will also collect quantitative data.What are the 3 primary case study? ›
He has helpfully characterised three main types of case study: intrinsic, instrumental and collective. An intrinsic case study is typically undertaken to learn about a unique phenomenon.Is case-control study a bias? ›
Case-control studies are typically prone to selection bias (g is true). Selection bias occurs if the recruited cases or controls are systematically different from the population of people they are intended to represent.Can you calculate risk ratio in case-control study? ›
As a result, risks, rates, risk ratios or rate ratios cannot be calculated from the typical case-control study. However, you can calculate an odds ratio and interpret it as an approximation of the risk ratio, particularly when the disease is uncommon in the population.What type of data is cross-sectional data? ›
Cross-section data is collected in a single time period and is characterized by individual units - people, companies, countries, etc. Some examples include: Student grades at the end of the current semester; Household data of the previous year - expenditure on food, unemployment, income, etc.Is a cross-sectional study an experimental design? ›
Cross-sectional studies are descriptive studies (neither longitudinal nor experimental). Unlike case-control studies, they can be used to describe, not only the odds ratio, but also absolute risks and relative risks from prevalences (sometimes called prevalence risk ratio, or PRR).What bias is in cross-sectional study? ›
A common type of selection bias is the nonresponse bias, which is usually encountered in cross- sectional survey studies with mailed questionnaires. A nonresponse bias occurs when the characteristics of nonresponders differ from responders.What are the pros and cons of cross-sectional study? ›
|Used to prove or disprove assumptions||Not used to analyze behavior|
|Cheap and quick||Useless for determining cause and effect|
|Multiple variables at the time of a data snapshot||Snapshot timing may not be representative|
Selection bias can be minimized in cross sectional studies by trying to contact those who cannot be contacted during the survey timings. It is worthwhile going through following lines in the endgame first (1): "Therefore, ownership of a phone and listing in the directory would have influenced inclusion in the study.Is a cross-sectional study a case series? ›
A case report is the description of the clinical story of a single patient. A cross-sectional study involves a group of participants on which data is collected at a single point in time to investigate the relationship between a certain exposure and an outcome.
Case control study inspects individuals by outcome/disease status. But, the retrospective cohort study inspects individuals by their exposure status.Is case study and case control study the same? ›
A case report is the description of the clinical story of a single patient, whereas a case-control study compares 2 groups of participants differing in outcome in order to determine if a suspected exposure in their past caused that difference.What is the difference between cross-sectional and retrospective study? ›
These studies can be seen as a variation of the cross-sectional design as they involve two sets of cross-sectional data collection on the same population to determine if a change has occurred. Retrospective studies investigate a phenomenon or issue that has occurred in the past.What is the difference between case-control and case series? ›
Case series have no control at all because only patients with a certain manifestation of outcomes are sampled (e.g., individuals with a disease or deceased individuals). In contrast, all case-control designs as well as self-controlled case-control designs have a control group.How do you analyze a case-control study? ›
Case-control studies produce the odds ratio to measure the strength of the link between exposure and the outcome. An odds ratio is the ratio of exposure probabilities in the case group to the odds of response in the control group. Calculating a confidence interval for each odds ratio is critical.What is the difference between a case and case study? ›
A case study is a qualitative research approach where multiple methods of data collection are used for a detailed examination of a single 'case'. A case is a single unit in a study. It can be a person or an organisation, like a clinic, police force, factory or a hospital.What are the limitations of a case-control study? ›
The main issues of concern with a case-control study are recall bias, its retrospective nature, the need for a careful collection of measured variables, and the selection of an appropriate control group.  These are discussed above in the disadvantages section.Are case-control studies experimental or observational? ›
Cohort studies and case control studies are two types of observational studies.What level of evidence is a case-control study? ›
There is no randomization and, as with a case series, datum is usually collected only once for each subject. For a case-control study, Level of Evidence = III. The case series study, which is usually retrospective, involves one group of patients (its cases).Why do we use cross-sectional study? ›
Cross-sectional designs are used for population-based surveys and to assess the prevalence of diseases in clinic-based samples. These studies can usually be conducted relatively faster and are inexpensive. They may be conducted either before planning a cohort study or a baseline in a cohort study.
Case-control studies have specific advantages compared to other study designs. They are comparatively quick, inexpensive, and easy. They are particularly appropriate for (1) investigating outbreaks, and (2) studying rare diseases or outcomes.What level is a cross-sectional study? ›
Cross sectional study designs and case series form the lowest level of the aetiology hierarchy. In the cross sectional design, data concerning each subject is often recorded at one point in time.Is cross-sectional study qualitative or quantitative? ›
Although the majority of cross-sectional studies is quantitative, cross-sectional designs can be also be qualitative or mixed-method in their design.Are case-control studies primary or secondary? ›
Unfiltered resources are primary sources that describe original research. Randomized controlled trials, cohort studies, case-controlled studies, and case series/reports are considered unfiltered information.