Dr Jenny McSharry, Health Psychologist and lecturer in the School of Psychology at the University of Galway, explains what systematic reviews are and why they are a particularly trustworthy source of health evidence.
Dr Jenny Mc Sharry was supported by an Evidence Synthesis Ireland Writing Mentorship (Grant code ESI-2021-001) and this blog post was originally posted on Evidently Cochrane.
When we are diagnosed with a new condition, experience a worrying symptom or need to make a health decision for ourselves or our loved ones, it can be hard to know where to turn. How often have we found ourselves unable to sleep, alone at 3AM, searching online for answers to questions we never thought we would have to ask? In times of anxiety and uncertainty, how can we know what health evidence to trust when we need it most?
When looking for reliable information to help with health decisions, reviews of evidence, in particular systematic reviews, can be a good place to start.
Systematic reviews of multiple studies can be useful for health decisions
Reviews that bring together and summarise the existing evidence are more reliable than one research study, or one person’s opinion or experience. Lots of things can affect the findings of a research study and we can have more confidence in reviews that bring together lots of studies. There is more information on why Personal experiences or anecdotes (stories) are an unreliable basis for assessing the effects of most treatments on the Evidently Cochrane blog.
Systematic reviews are seen as the gold-standard of research evidence. A systematic review is a type of review that tries to find, assess, and summarise all the evidence that meets pre-specified criteria to answer a specific question. Researchers who complete systematic reviews follow a number of key steps to make their findings more trustworthy.
Systematic reviews summarise evidence related to a specific question
Systematic reviews look for evidence that might help answer a specific question, and clearly outline this question from the start. For example, a systematic review Antihistamines for motion sickness was done to find out if medicines used for allergy symptoms (antihistamines) work and are safe in preventing travel (motion) sickness. The review summarises evidence on how antihistamines compare with dummy treatment (placebo), no treatment, and other medicines. This review is useful if you want to find evidence about medicines for travel sickness, but not if you want to know if you should avoid reading or drink ginger tea while travelling. When a review clearly states its focus, it helps us judge whether it is relevant and useful to us.
Systematic reviews aim to find all studies that meet certain criteria
Systematic reviews researchers try to find and include all studies relevant to a specific question. This is important, as we want all relevant information to be available to us when making a health decision. When using evidence from a review to decide if we should start a new treatment for example, we want to be sure that studies that found benefits, studies that found harms, and studies that found little or no difference, are just as likely to have been included.
Systematic review researchers try to find all relevant studies by clearly outlining the sources they will search, and by making a list of the criteria they will use to decide if studies should be included. For example, in our review Video calls for reducing social isolation and loneliness in older people, we wanted to find out if older people who used video calls to keep in touch with family and friends feel less lonely than those who did not. For this review, we needed to decide on what we meant by older adults, as the findings might be different for people aged 65 and older, or people aged 70 and older. For this review, we decided to include studies of people aged 65 and older and made it clear when reporting the findings that this was the group we had focused on.
Systematic review researchers clearly state the methods they will use to identify studies before starting the review. Other researchers should be able to use these methods and find the same set of studies.
Systematic reviews assess and report on the quality of evidence
When reading reviews of evidence, it is important to know how many relevant studies there are, but also to know if the studies are of good quality. Even if a number of studies find that a treatment is beneficial, we may not want to choose this treatment if we find out that the number of people included in the studies was very small, or that the way in which people were identified to take part in the studies was not clear. For a systematic review, researchers assess if the methods used in the included studies were appropriate. Researchers then make a judgement on the level of certainty we can have in the overall evidence, based on whether the included studies were done well or not.
Finding relevant systematic reviews: Where to look?
When it comes to trustworthiness, not all information we find is equal, and it is good to know that systematic reviews can be helpful in making health decisions. However, when feeling anxious about a health condition or symptom, or when trying to make an urgent health decision, it can be hard to know how to find relevant systematic reviews.
Cochrane is an independent global organisation that aims to provide high-quality evidence to inform decisions about health and care. Cochrane systematic reviews are published online in the Cochrane Library. The Cochrane Library is free to access for people in the UK and Ireland (and in many other countries). You can search by topic, and the reviews all have a Plain Language Summary. The blogs here on Evidently Cochrane aim to make health evidence easy to understand and use by providing summaries of Cochrane Reviews in context, often alongside patients’ and health professionals’ experiences. Evidently Cochrane includes an A-Z list of topics to make it easier to search for relevant evidence, and can be a good place to start when supporting family and friends or making choices about our own health.
What else can you do to work out if health information is trustworthy?
If you are interested in finding out more about health information and what we can trust, the blog, Making health decisions: things that can help, looks at some key things to consider to help us to make the right decision and to reduce the risk of regretting our choices later on. The blog also reminds us that summaries of research evidence are just one part of how we make health decisions, and our own preferences and values and the views of healthcare professionals are also very important. The blog includes lots of resources to help you think critically about health information and to support your decision-making.
- Learn more about health evidence in our free Cochrane Evidence Essentials resource
- Read more Evidently Cochrane blogs
- Join in on the conversation on Twitter with @jenmcsharry @CochraneUK or leave a comment on the blog