| Randomized clinical trial |
- Considered the gold standard for analyzing therapeutic interventions
- The study groups differ only in terms of the intervention factor: there is direct inference of causality
- Ideally, it is able to control selection bias and confounders that can affect the study outcome |
- Expensive and time consuming
- Not always feasible for ethical reasons
- The absence of blinding, especially when it cannot be applied, may directly affect the study result
- Subject to loss of patients during follow-up
- Generally evaluates specific disease scenarios
- Frequently conducted in academic scenarios, limiting the generalizability of the data |
| Nonrandomized clinical trial |
- Performed when an RCT would be ideal but is not possible (due to cost or unacceptability to patients or managers)
- Generally more practical than RCT
- Used when attempting to demonstrate the efficacy of a new treatment (phase IV studies) |
- Sample group is determined by convenience
- Causality is not directly inferred
- The effect of confounders may not be obvious |
| Before-and-after study |
- Quick and easy to conduct
- May be used to compare the ICU data or variables of a specific patient before and after an intervention is instituted
- Useful in interventions that are difficult to perform using a clinical trial (e.g., to evaluate the efficacy of isolation measures in the transmission of nosocomial infections by multiresistant germs) |
- Subject to the Hawthorne effect
- Simultaneous interventions may alter the studied outcome and the interval between the measurement of outcomes
- Subject to the “regression toward the mean” phenomenon |
| Cross-sectional study |
- Measures the prevalence of a particular outcome
- One of the first stages in investigating the cause of disease outbreaks
- May be used to analyze frequencies of risk factors and study outcomes
- Low cost and easy to conduct |
- Measures of exposure and disease are performed at the same time, which reduces the study’s ability to establish a causal association (association does not necessarily imply causation)
- Difficult to use for investigating low-prevalence conditions
- Appropriate when evaluating diagnostic tests or disease outbreaks |
| Case series |
- The characteristics of the study population are related
- Useful when characterizing rare disease scenarios with a diagnosis or treatment that is not clearly established in the literature (for example: the characterization of outbreaks of rare micro-organisms) |
- There is no comparison group (reference)
- Sampling is not representative of the studied population
- Causal associations cannot be evaluated |
| Case-control |
- Cheaper and faster (than cohort studies)
- Useful when investigating disease causes, especially when the incidence of the outcome (disease) is rare or has a long latency period, as the starting point is the outcome and a retrograde analysis of risk factors is performed |
- There is no way of knowing the incidence of the disease
- Prone to sampling bias among both cases and controls and to observation and recording bias |
| Cohort |
- Provides the best information about the etiology, incidence and natural history of a disease by starting with risk factors and analyzing subsequent outcomes
- Useful when risk factors have a low prevalence
- A cohort study should be used when it is not possible to conduct an RCT for ethical reasons |
- Expensive and requires long periods of observation
- Subject to loss to follow-up
- Unable to control all confounders
- Subject to discontinuity of patient follow-up
- Frequently presents results that are discordant with those of RCT that analyze the same research question |
| Systematic review with meta-analysis |
- Clinical research with the lowest evidence level
- Summarizes the results of primary studies using strategies that decrease the occurrence of random and systematic errors |
- The quality of a meta-analysis depends on the quality of the studies it includes
- The greater the heterogeneity of the included studies, the lower the reliability of the result obtained |
Different research designs and their characteristics in intensive care