Keller et al.19	Nakib et al.20
Country of origin	Taiwan	United States
Study design	Retrospective cohort	Prospective cohort
Year of publication	2012	2014
Cases	Cases of periodontitis were identified from the Longitudinal Health Insurance Database which included random sample of one million enrollees under the Taiwan National Health Insurance Program. The study included subjects with first diagnostic code of periodontitis between 1 January 2001 and 31 December 2004. Subjects in this study needed to have at least 2 diagnostic codes of periodontitis in the database.	Cases of periodontitis were identified from Nurses' Health Study cohort which was established in 1976. Subjects with self-reported history of periodontitis in the 1998 survey and were free of psoriasis at that time were included.
Comparators	Comparators were sex, age and urbanization-matched. They were randomly recruited from the same database.	Comparators were the rest of subjects in Nurses' Health Study cohort who did not have psoriasis and periodontitis.
Identification and verification of psoriasis	Presence of diagnostic code for psoriasis was used to identify the diagnosis of psoriasis. No further verification was made.	Self-reported physician–diagnosed psoriasis though health questionnaires that were mailed to the subjects every 2 years
Number of subjects (cases/comparators)	115,365 /115,365	11,358 /70,020
Percentage of female (cases/comparators)	52.1/52.1	100.0/100.0
Mean age in years (cases/comparators)	39.2/39.2	64.7/64.6
Follow up	Medical records were tracked for 5 years after index date	Until last health questionnaire in 2008
Confounder assessed	Income, urbanization and geographic region	Age, body mass index, smoking, alcohol and physical activity
Quality assessment (Newcastle – Ottawa scale)	Selection: 4 stars Comparability: 2 stars Outcome: 3 stars	Selection: 3 stars Comparability: 1 star Outcome: 3 stars