Introduction

Public health evidence is different to clinical evidence (1), because it involves a wide range of interventions, affecting complex populations rather than one individual. Throw in the confounding factors of social determinants of health and you realise that effectively finding the evidence for public health decision-making means you have to think out of the box. You are no longer searching for the evidence on clinical conditions, but instead the working/living environment, socioeconomic factors, such as education, which affects literacy and therefore choices, criminal activity, substance abuse, climate change, etc (2). This blog post will summarise the key steps to finding the evidence for public health, highlighting some key resources and useful tips.

Levels of evidence
There are different types of evidence available, and some are higher quality than others because of the rigorous methodology that has been applied. Systematic reviews, meta-analyses, evidence summaries, and guidelines are the highest levels of evidence, followed by randomised clinical trials, longitudinal/cohort studies, cross-sectional studies, case studies, and expert opinion. Most databases will let you limit your search by evidence type.

Formulating a searchable question
A searchable question outlines what you are looking for. It is brief and clear, identifying the problem, and exposure or intervention and the potential outcomes. An example of a searchable question is:

Does exercise in middle age reduce the risk of dementia?

Identifying search terms
Once you have a question, break it down in to concepts, to help focus your search strategy. There are a number of frameworks that can help you do this (3):

Because of the complexity of some public health questions, it may be that your question might not neatly fit in to any of these frameworks. The important thing is to identify the key concepts, to help you build your search strategy.

Draw up a template, and fill in the boxes with the key concepts from your searchable question. Use your subject expertise to identify additional terms:

Then, if you can, find some gold standard articles, as these can help you recognise more key terms. A gold standard article is an article relevant to your search topic, one that answers your search question, and can be used to inform your search strategy. As an expert in your subject area, you may already be aware of some relevant papers. An example of a gold standard paper for this question might be:

Morgan GS, Gallacher J, Bayer A, Fish M, Ebrahim S, Ben-Shlomo Y. Physical activity in middle-age and dementia in later life: findings from a prospective cohort of men in Caerphilly, South Wales and a meta-analysis. Journal of Alzheimers Disease, 2012, 31(3): 569-80.

This paper was written in 2012, and our search strategy may help us find more recent papers.

Choosing information sources
Internet search engines can be a useful starting point because they can help identify relevant search terms and gold standard papers. However, there is a lot of information on the Internet, and it is difficult to ascertain the quality of some of it. Therefore, two sources which are useful for finding good quality information quickly are TRIP Database and NICE Evidence Search, both of which are freely available and have handy navigation tools to make it easier to filter through the results to get the evidence that you need.

If you do not find anything using these two sources, then it will be necessary to carry out a bibliographic database search, which is slightly more complex. Bibliographic databases for public health include, Cochrane, PubMed and Embase (medicine, public health), CINAHL (nursing and allied health), Campbell Library (justice, welfare, education), Global Health, PsycInfo (psychology) LILACS (global health), ERIC (education), and SCIE Online (social care). More databases are available. Think about other health systems that might have similar situations, for example, when looking at the evidence for sugar tax, there was quite a bit of research set in Mexico.

Searching bibliographic databases
There are two ways to search a bibliographic database: free text and thesaurus.

Free text (also known as natural language) searching means that you enter your search term(s), and sometimes the database searches for those terms exactly as you have entered them, ignoring variances in spelling, such as behaviour and behaviour. Some databases allow you to use wildcards, such as “?”, e.g. behavio?r, so that it searches for either spelling. Truncation is another useful shortcut, as it lets you include various endings and plurals, e.g. adolescen* will search for adolescent or adolescence, and nurs* will search for nurse, nurses, and nursing. Asterisks (*) or dollar signs ($) are the most common truncation symbols, but use the database Help feature to get more advice on using each database most effectively. It is also possible to limit free text searches to just the Title and abstract of the article.

Thesaurus terms are also known as subject headings, descriptors, keywords, controlled vocabulary, or MeSH (Medical Subject Headings). When an article is added to the database, a set of terms are assigned to each record, to reflect the subject matter. This enables you to search much more effectively for papers on specific topics. However, thesaurus terms may not be immediately assigned upon entry to the database, so ideally you should search for both free text and thesaurus terms, to ensure that you pick up all the most recent, relevant papers. The thesaurus also has the option to Explode a term, so that any narrower, more specific terms are also included. For example, when exploding the thesaurus term for Exercise, the narrower thesaurus terms for walking, swimming, jogging, etc will also be included in the search.

Combining searches
Search for all the terms under each concept first, combining them with OR, and then combine all the concepts at the end, using AND. For example, search for the population/problem terms first, then the intervention terms, comparison/context terms (if applicable), and then the outcome terms. This method should retrieve relevant articles which answer your question.

When you search, it is good practice to search for each term separately, so that you can experiment with different combinations, if you do not find what you are looking for on your first attempt.

To test if your search strategy is effective, make sure that it is picking up the gold standard papers that you originally identified. To do this, search for the titles of all your gold standard papers, combining them with “or”, and then combine them with your final search string, using “and” to see how many of them are retrieved. If some are not retrieved, then look at the thesaurus terms they have used, and if any are missing from your search strategy, add them in.

If you find a totally relevant paper, but the others are less relevant, then see what other thesaurus terms and key words that paper has included, and build them in to your search strategy.

Summary
In conclusion, developing a search strategy is an evolving process, and takes time. All databases use the same principles, but have different interfaces, so make use of the “Help” feature, when in doubt about techniques to use.

This paper by Akobeng (4), while aimed at a clinical audience, provides an excellent introduction to the 5 step model for evidence based practice, including question formulation, searching for evidence, and critical appraisal.

Bibliography

  1. Sackett David L, Straus Sharon E, Richardson W Scott, Rosenberg William, Haynes R Brian. Evidence Based Medicine: How to Practice and Teach EBM. Edinburgh: Churchill Livingstone; 2000.
  2. National Collaborating Centre for Methods and Tools. A Model for Evidence-Informed Decision Making in Public Health. Ontario, Canada: McMaster University; 2017 http://www.nccmt.ca/pubs/FactSheet_EIDM_EN_WEB.pdf
  3. Davies K. Formulating the evidence based practice question: A review of the frameworks. Evidence Based Library and Information Practice. 2011;6(2).
  4. Akobeng AK. Principles of evidence based practice. Archives of Disease in Childhood. 2005;90(8):837-52 http://adc.bmj.com/content/90/8/837.full.pdf+html

Dr Caroline De Brun, DipLIS – Knowledge and Evidence Specialist, Public Health England.
To find out more, contact Caroline via email

Caroline De Brun

I started my career working in academic libraries before moving into medical librarianship, and have worked for the NHS since 1999, currently working as a Knowledge and Evidence Specialist for Public Health England. Throughout my career, I have focused mainly on health information literacy, evidence based practice, and knowledge management. I have worked in primary care, mental health, acute care, and on several national projects. I have extensive experience in teaching information skills to health professionals, and have co-authored the “Searching Skills Toolkit: Finding the Evidence”, which is aimed at helping health professionals find the best evidence. The topic of my doctoral thesis was collaborative working to facilitate access to consumer health information, and lately have been working with public librarians to help them support members of the general public searching for good quality health information.