However, efforts to link data are met by three problems:

1. Non-consent: Participants may refuse to allow survey teams to access their administrative records and link them to survey data.
2. Non-coverage: Even when participants give consent, it may not be possible to find them in the administrative register.
3. Missing values: Even when a participant gives consent and can be found on the administrative register, their records could be missing information on certain variables.

These problems can cause biases when analysing linked data. Using linked data from the Millennium Cohort Study (MCS), this project examined patterns of non-consent and non-coverage, and identifies weighting and imputation techniques that can adjust for biases.

Consent and coverage in data linkage is an under-researched area of survey methodology. However, it has become increasingly urgent to find solutions to these challenges as the demand for linked data sets rises.

This project was funded by the National Centre for Research Methods and the Economic and Social Research Council. It ran from April 2013 to September 2014.