Table 4.

Formulae to calculate the PPV, NPV, and sensitivity and specificity

	NMSC database	Medical record review
		yes	no
eg, First skin cancer ever	Yes	a	b	PPV = a/(a + b)
	No	c	d	NPV = c/(c + d)
		Sensitivity = a/(a + c)	Specificity = b/(b + d)

Notes: PPV, positive predictive value. The proportion of patients registered with ‘yes’ for a given variable in the database confirmed in the medical record (ie, the variable: diagnosis, according to the DPR) for variables: ‘diagnosis overall’, ‘treatment overall’, ‘size’, localization’ and ‘treatment date’ PPVs were the proportion of a given variable registered in the database confirmed in the medical record/DPR; NMSC, nonmelanoma skin cancer; NPV, The proportion of patients registered with ‘no’ for a given variable in the database confirmed in the medical record (ie, the variable; diagnosis, according to the DPR); Sensitivity, The proportion of patients registered with ‘yes’ for a given variable in the medical record, who were registered with a ‘yes’ in the database (ie, the variable diagnosis, according to the DPR); Specificity, The proportion of patients with ‘no’ for a given variable in the medical record, who were registered with a ‘no’ in the database (ie, the variable diagnosis, according to the DPR).

Abbreviations: DPR, Danish Pathology Registry; NMSC, nonmelanoma skin cancer; NPV, negative predicted value; PPV, positive predicture value.