The Australian Paradox

Professor Jennie Brand-Miller and Dr Alan Barclay

How do you assess the quality of scientific evidence?

How good is the evidence?

Each and every day, thousands of studies are published. Many of the headlines from the abstracts are published in the popular media, but how do you know if the “latest research” is worth worrying about? To help answer this, you need to know a little bit about some of the more common study designs.

Randomised controlled trials (RCTs) in humans

These are considered the “gold standard” research design as they can prove that intervention A improves health outcome B, while all other known factors (known as confounders) have been accounted for by randomisation. For example, two groups of people are fed two different diets and their health status is measured over a period of a few weeks to several years. Having a control group is vital, because it enables you to compare two different interventions.

Observational studies

Where large groups of people have a medical check-up, their dietary patterns are measured, and they are followed up regularly for long periods of time (e.g., 5-20 years)), are not as good as RCTs as it is not possible to control for all confounders, and our tools of observation (e.g., a food frequency questionnaire for measuring a person’s usual food intake) are imperfect.

Animal studies

Can be used to generate hypotheses, and to investigate physiological mechanisms.

In vitro studies

Are studies that are done in the laboratory using cells, or tissue cultures. They too can be used to generate hypotheses, and to investigate physiological mechanisms.

Systematic literature reviews and meta-analyses.

A systematic literature review is the process of carefully searching medical databases (e.g., PubMed) with pre-determined search terms looking for all of the research published on a particular topic over a long period of time. Once all studies have been identified, researchers then go through each papers reference lists to make sure as best as possible that they have not missed any additional evidence. The data from each paper is then extracted and the results summarised in a table. The quality of each study is also rated. Conclusions can be drawn from the summarised data.

When 3 or more studies on a particular health outcome have been published, it may be possible to combine the data using a process known as meta-analysis. The outcome data from each study is entered into specialised software and weighted according to the study size and statistical significance. A final summary statistic is given that indicates whether an intervention is effective, and if so, how effective. Systematic literature reviews and meta-analyses of randomised controlled trials are considered the highest level of evidence. Cochrane reviews are a good example of this method. You can also do systematic reviews and meta-analyses of observational studies. However, because the underlying study design is not as robust as the randomised controlled trials, they are not considered to be as high a level of evidence as a Cochrane review, for example.

What does it all mean?

If the latest study broadcast in the news is a systematic literature review and meta-analysis of randomised controlled trials then the results are definitely worth talking notice of if the people involved are similar to you, and live under similar circumstances. If the latest study broadcast in the news is a systematic literature review and meta-analysis of observational studies then the results are definitely interesting, but a randomised controlled trial in humans that studied the same effect would be necessary to prove that the relationship was causal. If the latest study broadcast in the news is based on an animal study or in vitro study it’s highly likely that more research in humans is needed in order to prove the hypothesis.