Effects of aggregation of drug and diagnostic codes on the performance of the high-dimensional propensity score algorithm: an empirical example

Table 5 Changes of prevalence, covariate-exposure and covariate-outcome relations when we aggregated potential confounders, clopidrogel and warfarin from level 5 to levels 4, 3, 2 and 1 of the Anatomical Therapeutic Chemical (ATC) classification

Dictionary/Level	Code	Description	Frequency	Frequency type	Covariate exposure risk ratio	Covariate-outcome risk ratio	Prevalence in both groups	Included in lower level
Generic drug		Clopidrogrel	218	once	1.5	3.8	0.012
Generic drug		Clopidrogrel	218	sporadic	1.4	2.9	0.012
Generic drug		Warfarin	319	once	1.6	2.0	0.017
Generic drug		Warfarin	319	sporadic	1.7	1.3	0.017
ATC level 5	B01AC04	Clopidrogrel	218	once	1.5	3.8	0.012
ATC level 5	B01AC04	Clopidrogrel	218	sporadic	1.4	2.9	0.012
ATC level 5	B01AA03	Warfarin	319	once	1.6	2.0	0.017
ATC level 5	B01AA03	Warfarin	319	sporadic	1.7	1.3	0.017
ATC level 4	B01AC	Platelet aggregation inhibitors excluding heparin	253	once	1.5	3.3	0.013
ATC level 4	B01AC	Platelet aggregation inhibitors excluding heparin	253	sporadic	1.5	2.5	0.013
	B01AC04	Clopidrogrel	218					Yes
	B01AC05	Ticlopidine	1				0.000	No
	B01AC07	Dipyridamole	6				0.000	No
	B01AC23	Cilostazol	25				0.000	No
	B01AC30	Combinations	11				0.000	No
ATC level 4	B01AA	Vitamin K antagonists	319	once	1.6	2.0	0.017
ATC level 4	B01AA	Vitamin K antagonists	319	sporadic	1.7	1.3	0.017
ATC level 3	B01AA03	Warfarin	319					Yes
ATC level 3	B01A	Antithrombotic agents	637	once	1.5	1.5	0.034
ATC level 2	B01A	Antithrombotic agents	637	sporadic	1.6	2.0	0.034
ATC level 2	B01	Antithrombotic agents	637	once	1.5	1.5	0.034
ATC level 2	B01	Antithrombotic agents	637	sporadic	1.6	2.0	0.034
ATC level 1	B	Blood and blood forming organs	1049	once	1.4	1.4	0.056
ATC level 1	B	Blood and blood forming organs	1049	sporadic	1.4	1.9	0.056
ATC level 1	B	Blood and blood forming organs	1049	frequent	1.5	2.0	0.025

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