Categorisation of continuous risk factors in epidemiological publications: a survey of current practice

Table 7 Estimation and statistical testing by analysis type^a.

	Analysis type
	Continuous (n = 8)	Categorical (n = 29)	Both (n = 21)	Overall (n = 58)
Type of estimate - n
Continuous	7	0	16	23 (40%)
By group for all groups	0	4	6	10 (17%)
By group relative to ref group	0	26	12	38 (66%)
Other	1^b	1^c	3^d	5 (9%)
Type of statistical test - n
Continuous	8	0	19	27 (47%)
Score trend test	0	11	1	12 (21%)
Median/mean trend	0	7	1	8 (14%)
Pairwise	0	17	9	26 (45%)
Global	0	3	6	9 (16%)
Other	0	0	1^e	1 (2%)

For the 58 articles with a continuous risk factor.
^a More than one estimate type and more than one statistical test is possible: 40 (69%) articles had one type of estimate (8 from 'continuous', 27 from 'categorical' and 5 from 'both') whilst 18 (31%) articles had two types of estimate (2 from 'categorical' and 16 from 'both'); 35 (60%) articles had one type of statistical test (8 from 'continuous', 20 from 'categorical' and 7 from 'both'); 21 (36%) articles had two types of statistical test (9 from 'categorical' and 12 from 'both') and 2 (3%) articles (from 'both') had three types of statistical test.
^b A continuous analysis estimate given as difference between 90^th and 10^th percentile.
^c Reference group is the background population overall i.e. standardised incidence.
^d One article gave hazard ratios per one category increase, another article compared 1^st and 4^th quartiles only, the final article reported the mean by categories.
^e A t-test comparing means in two outcome groups.