Table 1.

Key findings showing the characteristics of categorisation amongst the exposure variables in epidemiological studies

Characteristics of categorisation	% of articles & CI regions
Prevalence of categorisation	61% (CI = 39%, 80%)
Decision informing categorisation
Hypothesis-driven categories	7% (CI = 0%, 34%)
Unknown (reasons not provided in the articles)	93% (CI = 66%, 100%)
Criteria used for categorisation
Established external criteria (e.g., WHO standards)	14% (CI = 2%, 43%)
Arbitrary grouping	29% (CI = 8%, 58%)
Equally spaced interval grouping	36% (CI = 13%, 65)
Quantile grouping	21% (CI = 5%, 51%)
Number of categories used amongst grouped exposures
2	7% (CI = 0%, 34%)
3	7% (CI = 0%, 34%)
4	29% (CI = 8%, 58%)
5	29% (CI = 8%, 58%)
6	14% (CI = 2%, 34%)
10	14% (CI = 2%, 34%)
Proportion of trend testing	57% (CI = 29%, 82%)