Table 2: Linkage stage 1:8 step deterministic algorithm for linking the national pupil database to the personal demographic service.
| Step | First name | Surname | Date of birth | Sex | Postcode* |
|---|---|---|---|---|---|
| 1** | Exact | Exact | Exact | Exact | Exact |
| 2 | Soundex | Soundex | Exact | Exact | Exact |
| 3 | 1st character | Characters 1–3 | Exact | Exact | Exact |
| 4 | 1st character | Characters 1–3 | Exact | Exact | |
| 5 | Exact | Exact | Exact | ||
| 6 | Partial | Exact | Exact | ||
| 7 | Exact | Exact | Exact | Exact | |
| 8 | 1st character | Characters 1–3 | Exact | Exact |
Notes: * Full postcode (e.g. LS0 0AA). ** Step 1 was repeated by NHS Digital but allowing an NPD record to link to many PDS records. The objective of repeating this modified step 1 was to remove potential duplicate HESIDs for the same pupil. See details in Supplementary Appendix 4. Exact refers to exact linking; Partial refers exact linking but using month and year of birth only; Soundex refers to the Structured Query Language (SQL) algorithm that converts an alphanumeric string to a four-character code that is based on how the string sounds when spoken. NPD = National Pupil Database; PDS = Personal Demographic Service.