Have you ever wanted to feel more confident about interpreting the findings of clinical trials and reviews? We’re here to help with a new series of blog posts aiming to help you get to grips with understanding and interpreting biomedical research.

This month’s post focuses on 95% confidence intervals.

What is a confidence interval?

Confidence intervals can be calculated for most kinds of statistical test, e.g. p values, t tests, measures of risk and numbers needed to treat. They provide an indication of the strength of evidence e.g. for the effectiveness of a treatment. Confidence intervals define a range of values within which you can expect the statistical results of a study to fall.

If a study is carried out once it will obviously end with one set of results, and one set of statistics calculated from those results, e.g. a p value. If that study were to be repeated over and over again, you’d expect the results to be slightly different every time –if only because there will always be some variation.

So, you might find a different p value if a study were repeated. Statistics such as p values are often used to try and determine whether an intervention has a statistically significant effect, and it’s possible that one repeat of a study would find a non-statistically significant p value, even if the original study findings had been statistically significant. Confidence intervals can help you to understand how likely this is to happen.

What do confidence intervals mean?

Confidence intervals provide a range of values within which you would expect the results of a study to fall, were it to be repeated. Although the same trial repeated hundreds of times would not yield the same results every time, on average the results would be within a certain range.  95% confidence intervals are often calculated as standard. A 95% confidence interval means that there is a 95% chance that the true size of the intervention effect will lie within this range – so if a study were repeated there is a 95% chance that it would produce a result falling somewhere within the range defined by the confidence interval.

Find out more

In the next of this regular series of posts on interpreting medical statistics we’ll look at another common statistical technique, odds ratios. In the meantime the Library provides you with access to range of resources and papers on confidence intervals and other statistics. A short selection of further reading is given below.

Davies, Huw T.O., Crombie, Iain K. (2009) What are confidence intervals and p-values? Hayward Medical Communications

Sedgwick, Philip (2014). “Understanding confidence intervals” BMJ 2014; 349 :g6051

Don’t forget, we also run regular training sessions on critical appraisal for study types such as RCTs, systematic reviews and qualitative research.

Sources

Greenhalgh, Trisha (2014). How to read a paper : The basics of evidence-based medicine (5th ed.). Chichester: Wiley ; BMJ Books.

We have an exciting medical research resource for you to try! You may already be familiar with the freely available TRIP database, which can be accessed from any PC without the need for a log in or subscription. TRIP Pro is an enhanced, subscription version of the TRIP database, and is now freely available to Royal Free NHS London Foundation Trust staff. You can access TRIP Pro via any Royal Free networked computer.  If you have any problems accessing Trip, please contact the Medical Library.

If you haven’t used either TRIP Pro, or the freely available version of TRIP before, here’s a quick introduction:

What is Trip?

TRIP (which stands for Translating Research Into Practice) is a clinical search engine covering over 150 health resources, including indexing of content from sources such as  NICE, PubMed, the Cochrane Library and BMJ. Its main purpose is to act as a search engine for the most accurate and up to date clinical evidence. TRIP provides information on guidelines, systematic reviews, controlled trials, regulatory guidance, patient information leaflets, and e-books.

How is TRIP Pro different?

As well as all of the above features of the free version of TRIP, with TRIP Pro you’ll also have access to over 100,000 extra systematic reviews, millions of extra full-text articles, and the ability to search through ongoing clinical trials. You’ll also gain access to a database of medical images and videos.

Some hints and tips:

Sort by clinical area

Refine your search by articles which focus on your specialism – just click ‘Clinical Area’ in the ‘Refine by’ area on the right-hand side of the screen. E.g. a search for “asthma” can be refined by paediatrics, geriatrics, oncology, etc.

PICO Search function

This function allows you to break down a clinical question into four specific parts – Patient, Intervention, Comparison and Outcome. This allows to focus your search even further, and provides you with a useful structure for your clinical research.

Exporting results

To make writing journal articles a little easier, TRIP can automatically export your search results into a reference management service of your choice. Just select the desired search results, download them and upload them onto your chosen reference management service:

If you’d like to find out more about searching TRIP, some handy video guides explaining the basics of using TRIP can be found here.

Happy searching, and please send any questions or feedback to rlibrary@ucl.ac.uk or tweet us @RFHMLibrary