Table 2.
Criteria for assessing the quality of the study adapted from the SIGN checklist
| Section 1: internal validity | Cohort study | Case–control study | RCT/CCT |
|---|---|---|---|
| In a well-designed study | The study has addressed an appropriate and clearly well-defined research question. | The study has addressed an appropriate and clearly well-defined research question. | The study has addressed an appropriate and clearly well-defined research question. |
| Selection of participant | Two groups being studied are selected from source population and that are comparable | Cases and controls are taken from comparable populations | The assignment of participants to intervention/treatment groups is randomised |
| The study indicates how many of the people asked to take part did so in each of the groups being studied | The same inclusion/exclusion criteria are used for cases and controls | An adequate concealment method is used | |
| The likelihood that some eligible participants might have the outcome at the time of enrolment is assessed and taken into account in the analysis | What percentage of each group (cases and controls) participated in the study? | The design keeps participants and investigators ‘blind’ about treatment allocation | |
| What percentage of individuals or clusters recruited into each arm of the study dropped out before the study was completed? | Comparison is made between participants and non-participants to establish their similarities or differences | The treatment and control groups are similar at the start of the trial | |
| Comparison is made between full participants and those lost to follow-up by exposure status | Cases are clearly defined and differentiated from controls | The only difference between groups is the treatment under investigation | |
| Assessment | The outcomes are clearly defined | Measures have been taken to prevent knowledge of primary exposure influencing case ascertainment | All relevant outcomes are measured in a standard, valid and reliable way |
| The assessment of outcome is made blind to exposure status. If the study is retrospective, this may not be applicable | Exposure status is measured in a standard, valid and reliable way | All the participants are analysed in the groups to which they were randomly allocated | |
| Confounding | The main potential confounders are identified and taken into account in the design and analysis | The main potential confounders are identified and taken into account in the design and analysis | |
| Statistical analysis | CIs are provided | CIs are provided | CIs are provided |
| Section 2: overall assessment of the study | How well was the study conducted to minimise the risk of bias or confounding? | How well was the study conducted to minimise the risk of bias or confounding? | How well was the study conducted to minimise bias? |
| Methodology used for evaluation and the statistical power of the study, there is clear evidence of an association between exposure and outcome | Methodology used for evaluation and the statistical power of the study, there is clear evidence of an association between exposure and outcome | Methodology used for evaluation and the statistical power of the study, there is clear evidence of an association between exposure and outcome | |
| The results of this study are directly applicable to the patient/intervention groups targeted by this study | The results of this study are directly applicable to the patient/intervention groups targeted by this study | The results of this study are directly applicable to the patient/intervention groups targeted by this study |
CCT, controlled clinical trial; CI, confidence interval; RCT, randomised controlled trial; SIGN, Scottish Intercollegiate Guidelines Network.