Evidence Based Medicine
Study Methodology
Which of the following is NOT a disadvantage of a cohort study used to investigate the relationship between exposure to a risk factor and a future outcome:
Answer:
Advantages:- ideal for studying associations between an exposure and an outcome when the exposure is uncommon
- the time sequence of events can be assessed
- they can provide information on a wide range of disease outcomes
- the absolute and relative risk of disease can be measured directly
- they can give a direct estimation of disease incidence rates
- costly and can take long periods of time if the outcome is delayed
- subject to subject-selection and loss to follow-up bias
- large sample size required for rare outcome of interest so it is not useful for rare diseases
- prone to confounding
Observational Studies
Evidence Based Medicine / Study Methodology
Last Updated: 10th April 2024
Case-control Study
A case-control study is a longitudinal, retrospective, observational study which investigates the relationship between a risk factor and one or more outcomes. This is done by selecting patients who already have a specific disease (cases), matching them to patients who do not (controls) and then collecting data from the patients to compare past exposure to a possible risk factor. The usual outcome measure is the odds ratio.
Advantages:
- relatively quick
- relatively cheap and easy to perform
- particularly suitable for studying associations between an exposure and an outcome when the outcome is uncommon or if the outcome occurs decades after exposure
- a wide range of risk factors can be investigated in each study
Disadvantages:
- subject to recall bias
- unlike in a whole population study, absolute risk cannot be quantified
- temporal relationship between exposure and outcome can be difficult to establish
- unsuitable for rare risk factors
- prone to confounding
Cohort Study
A cohort study is a longitudinal, typically prospective, observational study that follows a defined group (cohort) matched to unexposed controls for a set period of time and investigates the effect of exposure to a risk factor on a particular future outcome. The usual outcome measure is the relative risk (risk ratio).
Advantages:
- ideal for studying associations between an exposure and an outcome when the exposure is uncommon
- the time sequence of events can be assessed
- they can provide information on a wide range of disease outcomes
- the absolute and relative risk of disease can be measured directly
- they can give a direct estimation of disease incidence rates
Disadvantages:
- costly and can take long periods of time if the outcome is delayed
- subject to subject-selection and loss to follow-up bias
- large sample size required for rare outcome of interest so it is not useful for rare diseases
- prone to confounding
Cross-sectional Study
The above study designs are longitudinal where a sample of individuals are investigated at different time points.
Cross-sectional studies aim to provide data about population health, normal ranges of biological parameters, and disease prevalence or severity by observing the entire population, or a representative subset, at a single point in time.
In this kind of study we can determine prevalence (total number of cases of a condition existing within a population).
Cross-sectional studies are relatively simple and quick to perform and can be used to study multiple outcomes, but are subject to confounding and recall bias and are not suitable for studying rare diseases. Cross-sectional studies cannot be used to assess causation or to consider trends over time.
Comparison of Observational Studies
| Study | Case-control | Cohort | Cross-sectional | Interventional |
|---|---|---|---|---|
| Features | Retrospective, longitudinal, observational study used to investigate past exposure to a risk factor on a current outcome, usual outcome measure = odds ratio | Prospective, longitudinal, observational study used to investigate the effect of an exposure to a risk factor on a future outcome, usual outcome measure = risk ratio | Cross-sectional observational study used to provide a snapshot of population health, biological parameters and disease prevalence by observing the entire population or a representative subset at a single point in time | Randomised control trials (RCTs) |
| Advantages | Relatively quick, cheap and easy to perform; wide range of risk factors can be investigated; suitable for studying a rare outcome or when the outcome is delayed | Suitable for studying a rare exposure; time sequence of events can be assessed; wide range of disease outcomes can be studied; absolute and relative risk of disease can be measured directly; can give a direct estimate of disease incidence rates | Relatively simple and quick to perform; can study multiple outcomes; relatively cheap | Gold standard for studying treatment effects; provide reliable measure of efficacy; allow for meta-analysis; can control for confounding variables |
| Disadvantages | Subject to recall bias; absolute risk cannot be quantified; temporal relationship cannot be established; unsuitable for rare risk factors | Costly and can take long periods of time if outcome is delayed; subject to subject selection and loss to follow-up bias; not suitable for rare diseases | Subject to recall bias; not suitable for rare diseases; cannot be used to assess causation or to consider trends over time | Difficult, time-consuming and expensive to set up; may be ethical problems in giving two different treatments to two groups |
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- Biochemistry
- Blood Gases
- Haematology
| Biochemistry | Normal Value |
|---|---|
| Sodium | 135 – 145 mmol/l |
| Potassium | 3.0 – 4.5 mmol/l |
| Urea | 2.5 – 7.5 mmol/l |
| Glucose | 3.5 – 5.0 mmol/l |
| Creatinine | 35 – 135 μmol/l |
| Alanine Aminotransferase (ALT) | 5 – 35 U/l |
| Gamma-glutamyl Transferase (GGT) | < 65 U/l |
| Alkaline Phosphatase (ALP) | 30 – 135 U/l |
| Aspartate Aminotransferase (AST) | < 40 U/l |
| Total Protein | 60 – 80 g/l |
| Albumin | 35 – 50 g/l |
| Globulin | 2.4 – 3.5 g/dl |
| Amylase | < 70 U/l |
| Total Bilirubin | 3 – 17 μmol/l |
| Calcium | 2.1 – 2.5 mmol/l |
| Chloride | 95 – 105 mmol/l |
| Phosphate | 0.8 – 1.4 mmol/l |
| Haematology | Normal Value |
|---|---|
| Haemoglobin | 11.5 – 16.6 g/dl |
| White Blood Cells | 4.0 – 11.0 x 109/l |
| Platelets | 150 – 450 x 109/l |
| MCV | 80 – 96 fl |
| MCHC | 32 – 36 g/dl |
| Neutrophils | 2.0 – 7.5 x 109/l |
| Lymphocytes | 1.5 – 4.0 x 109/l |
| Monocytes | 0.3 – 1.0 x 109/l |
| Eosinophils | 0.1 – 0.5 x 109/l |
| Basophils | < 0.2 x 109/l |
| Reticulocytes | < 2% |
| Haematocrit | 0.35 – 0.49 |
| Red Cell Distribution Width | 11 – 15% |
| Blood Gases | Normal Value |
|---|---|
| pH | 7.35 – 7.45 |
| pO2 | 11 – 14 kPa |
| pCO2 | 4.5 – 6.0 kPa |
| Base Excess | -2 – +2 mmol/l |
| Bicarbonate | 24 – 30 mmol/l |
| Lactate | < 2 mmol/l |
