In association with

  • Bristol, North Somerset and South Gloucestershire Integrated Care Board
  • West of England Academic Health Science NetworkWest
  • National Institute for Health Research



A priori

A priori comparisons are planned in advance of any data analysis. They are more reliable than post-hoc comparisons.

Absolute risk reduction (ARR)

The difference in the event rate between control group (CER) and treated group (EER): ARR = CER – EER.

Allocation concealment

Used when randomising participants to treatment groups in an RCT. Allocation is concealed when neither the participants nor the researchers know or can predict to which group in a study (control or treatment) the patient is assigned. Allocation concealment takes place before the study begins as patients are being assigned. It is a sign of a robust study.

AMED (Allied and Complementary Medicine Database)

A unique bibliographic database produced by the Health Care Information Service of the British Library. It covers a selection of journals in complementary medicine, palliative care, and several professions allied to medicine. Covers the years from 1985 to present and is updated monthly.


To examine associations between variables—these may be hypothesised causal or therapeutic relations.


A statistical dependence between two or more events, characteristics or other variables. An association may be fortuitous or may be produced by various other circumstances; the presence of an association does not necessarily imply a causal relationship

Average – Mean, Median, Mode – ways of expressing a central tendency

  • Mean – The average value, calculated by adding all the observations and dividing by the number of observations.
  • Median – Middle value of a list. If you have numbers 2, 3, 4, 5, 6, 7, and 8, the median is 5. Medians are often used when data are skewed, meaning that the distribution is uneven. In that case, a few very high numbers could, for instance, change the mean, but they would not change the median. Other definitions include the smallest number such that at least half the numbers in the list are no greater than it. If the list has an odd number of entries, the median is the middle entry in the list after sorting the list into increasing order. If the list has an even number of entries, the median is equal to the sum of the two middle (after sorting) numbers divided by two. The median can be estimated from a histogram by finding the smallest number such that the area under the histogram to the left of that number is 50%. But all mean the same thing in the end.
  • Mode – For lists, the mode is the most common (frequent) value.

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