Detecting referral and selection bias by the anonymous linkage of practice, hospital and clinic data using Secure and Private Record Linkage (SAPREL): case study from the evaluation of the Improved Access to Psychological Therapy (IAPT) service

Table 1 Missing, temporary or duplicate unique identifiers (NHS number) by practice

Practice number	List size	Duplicate or default NHS number	Missing NHS Number	Total of NHS Number problems	Missing Forename or Surname	Missing Post-code	Demographic data errors	TOTAL ERRORS	Errors as % List size
	N	n	n	n	%	n	n	n	%
2	13552	0	145	145	1.07	2	26	173	1.28
14	7190	68	0	68	0.95	0	1	69	0.96
20	10354	61	0	61	0.59	0	0	61	0.59
15	5189	49	0	49	0.94	0	5	54	1.04
11	7611	52	0	52	0.68	0	0	52	0.68
13	11828	45	0	45	0.38	0	2	47	0.4
5	6875	6	1	7	0.1	0	38	45	0.65
19	4715	38	0	38	0.81	0	4	42	0.89
4	8951	36	0	36	0.4	0	0	36	0.4
12	5971	31	0	31	0.52	1	3	35	0.59
1	6661	2	0	2	0.03	0	28	30	0.45
17	7338	23	0	23	0.31	0	2	25	0.34
7	9508	0	14	14	0.15	0	10	24	0.25
6	7951	4	1	5	0.06	0	8	13	0.16
18	3740	11	0	11	0.29	0	2	13	0.35
9	4466	4	7	11	0.25	0	0	11	0.25
16	14708	6	0	6	0.04	0	0	6	0.04
3	10049	0	2	2	0.02	0	0	2	0.02
8	2092	0	0	0	0	0	0	0	0
10	3614	0	0	0	0	0	0	0	0
Total	152363	436	170	606	0.4	3	129	738	0.48

ISSN: 1472-6947