Table 2. Number (%) of records with missing nhs number or postcode by birth year (inpatient hospital episodes from 1998 to 2015).

		Birth year (for records with day of birth 13th and 28th of each month in these years)
		1992 (n = 100,443)			1998 (n = 120,470)			2005 (n = 106,450)			2012 (n = 89,896)
Record characteristics		NHS number (%)	Postcode (%)		NHS number (%)	Postcode (%)		NHS number (%)	Postcode (%)		NHS number (%)	Postcode (%)
Overall	n	43.8	3.8	n	4.6	12.3	n	5.3	30.7	n	0.7	47.3
Sex
Missing	138	15.9	1.4	166	66.3	16.9	27	55.6	33.3	19	10.5	57.9
Male	60,285	4.2	3.8	54,779	37.6	13.0	47,939	5.5	33.0	40,849	0.7	50.4
Female	40,020	7.7	3.9	65,525	35.6	11.7	58,484	4.9	28.7	49,028	0.7	44.7
Ethnic group
Missing	18,988	11.3	3.9	48,098	53.5	16.9	19,938	8.5	47.5	7,966	2.1	40.6
White	70,455	3.7	4.0	59,977	24.0	9.3	67,887	4.2	26.1	62,037	0.5	47.6
Mixed	1,264	4.4	4.0	1,023	4.7	2.1	2,755	4.3	26.7	3,876	0.6	51.6
Asian	4,643	7.8	2.6	5,529	25.8	7.2	8,473	4.1	28.6	9,027	0.6	49.4
Black	3,103	6.9	2.2	2,909	33.2	6.6	4,665	6.4	32.0	3,963	1.2	50.0
Chinese/Other	1,990	14.2	2.7	2,934	49.6	16.1	2,732	7.4	29.3	3,027	1.6	44.1
Socio-economic group
Missing	3,836	8.2		14,784	61.7		32,644	6.5		42,523	0.4
Low deprivation	66,956	5.1		75,884	32.3		51,755	4.1		33,525	0.8
High deprivation	28,921	5.3		29,438	34.3		21,613	4.6		13,601	0.9
Foreign	620	57.6		341	80.6		324	78.7		214	43.5
No fixed abode	110	21.8		23	39.1		114	14.0		33	0.0
Episode type
Other episode				83,356	17.1	2.8	66,454	4.5	2.7	45,200	1.1	2.5
Birth episode				37,114	80.1	32.1	39,996	6.3	77.0	44,696	0.3	92.6

Note. Missing data on postcode refer to missing after excluding invalid or communal postcodes, and postcodes denoting ‘no fixed abode’ or foreign patients. The ‘mixed’ ethnic group was not recorded until April 2001 but appears in the 1992 and 1998 cohorts if taken from episodes from 2001 onwards. Proportions of missing data for local patient ID within provider are very small (<0.1%) and not shown here. High deprivation is defined as the most deprived quintile of IMD2004