Table 1 Eligibility criteria and rationale

1. Is a primary quantitative research study.

Although qualitative work can provide important insights into how systems operate, only quantitative studies provide estimates of interest at a population level. Mixed-methods studies were eligible if the quantitative component met the eligibility criteria. ‘Primary’ research was any research that used de novo data collection or analysis of record-level administrative data.

2. Has an educational outcome (i.e. attainment, exclusion, absenteeism, quality or type of school or participation in education beyond compulsory age).

This broad range of eligible educational outcomes was selected to include all facets of educational experience and success that are considered important for children in contact with CSC in current UK policy, as evidenced by the Department for Education statutory guidance on promoting the education of looked-after children and those previously looked-after. [9]

3. Main exposure is referral to CSC (any contact, including, e.g., ‘child in need’ assessments and provision, child protection investigations and care).

Exposure to CSC was also defined broadly as we made no a priori assumptions as to how studies would measure this.

4. Has a concurrent comparison group of the general population/children with no contact with CSC.

Uncontrolled studies do not provide an estimate of associations.

5. Sample or population studied is age < 18.

This study only examined childhood education.

6. Any study design.

We include all study designs, subject to the above criteria (e.g. cohort and cross-sectional designs); we did not expect any randomised-controlled trials.

7. English-language only.

Limited by the investigators’ languages. Only studies on UK populations were included (criterion 8) so this is unlikely to have been a biasing factor.

8. Conducted in any UK population.

Differences in social policy render cross-country comparisons difficult and international studies provide limited information regarding outcomes of children in the UK.

9. Conducted in 1991 onwards.

The law regarding the welfare of children in England was overhauled by the Children Act 1989, which came almost fully into force on 14 October 1991 (similar provisions were latterly enacted in the other three UK countries).

10. In a peer-reviewed source or not.

We were aware a priori that some studies in this area are published as non-peer-review reports. We did not therefore limit our search to peer-reviewed sources only.