4. Step 1: Determine the level of evidence

4.2. Is the study design appropriate to answer your question?

4.2 Is the study design appropriate to answer your question?

As we learnt in Ask, there are several question types we can pose. These questions can, in turn, be answered by a number of different study types.

The table below shows which study types provide the most robust evidence for different question types.

Table 9: Study types and levels of evidence

This table has been adapted and simplified from the Oxford Centre for Evidence-based Medicine Levels of Evidence table (2009). Find out more about the Oxford Centre for Evidence-based Medicine's Levels of Evidence tables .

Remember: For all question types, meta-analysis and systematic reviews are usually more robust than individual studies.

In your clinical decision-making, you should rely on the most robust evidence available; you need to determine the level of evidence a paper provides in answering your clinical question. You may also need to accept that the 'best available' evidence may be lower down in this table than you might prefer; there may only be a few individual case reports rather than a systematic review. But take heart – some evidence is better than none!

Remember, determining the level of evidence is only the first step in appraising your paper. Within each ‘level of evidence’, further appraisal of the study methods and reporting may reveal that the evidence is not as robust as you first thought… On the contrary, a paper which sits lower on the ‘level of evidence’ table may provide more robust evidence.

Table 10: Examples of the most robust study types for different types of clinical questions

Type of Question Example question Study type that will best answer the question*


In [dogs with osteoarthritis], does [supplementation with glucosamine and chondroitin] compared to [no supplementation] [reduce lameness]?

Randomised controlled trial, cohort study

Prognosis and Incidence

In [flat-coated retrievers with cutaneous lymphoma], does [being a male] compared with [being a female] affect [average life expectancy]?

Cohort study

Aetiology and Risk

In [ferrets], is [general anaesthesia by triple injectable agent] compared with [general anaesthesia by induction and inhalational agent] associated with [an increased risk of death]?

Cohort study, Case-control study, Cross- sectional study


In [lactating dairy cattle] does [milk ELISA] compared with [serum ELISA] have [a better sensitivity and specificity for diagnosing fascioliasis]?

Diagnostic test validation study


In [adult racehorses] what is the [prevalence of laryngeal neuropathy] in winter?

Cross-sectional study

*For all question types, meta-analysis and systematic reviews are more robust than individual studies.