Table 2: Potential sources of bias in evaluations of FNP in England and Scotland using linked administrative data and information needed to assess their likely extent.

Bias	Description	Impact on effect estimates	Information needed to avoid or assess likely bias
Indication bias due to FNP nurses deciding which mothers to approach (unmeasured confounding)	Family nurses prioritise the more vulnerable mothers among those meeting eligibility criteria, and so those enrolled may have been more likely than those not enrolled to experience adverse outcomes.	Underestimation of the effect of the intervention.	Knowledge of which characteristics prioritised for enrolment in each site (including start and end dates of these prioritisation characteristics); availability of data on these characteristics and other important maternal characteristics for adjustment purposes
Misclassification bias of eligibility for FNP	In analyses, mothers may have been assigned to different groups than the ones they should be in, because eligibility is incorrectly defined.	Bias in either/both directions: random misclassification is likelyto underestimate the intervention effect, but bias in misclassification may under- or over-estimate intervention effect.	Detailed recording of programme meta-data including site activity dates and geography, in order to correctly define eligible groups of mothers who were and were not enrolled or eligible for the intervention.
Consent bias for enrolment in FNP	Mothers who were offered the intervention but who declined may have been different to those who were not offered the intervention.	Bias in either/both directions. Those who were offered the intervention but who declined may be a mixture of the most vulnerable and the least vulnerable mothers.	Individual-level or aggregate data on characteristics of all mothers-to-be offered enrolment, and those who declined vs. who accepted enrolment.
Linkage bias	Linkage error (e.g. missed links or false links*) can mean that subgroups of the population were differentially excluded from the analysis cohort, or had missing data on variables obtained through linkage. Missed links can also lead to misclassification bias (see above).	Bias in either/both directions. It is difficult to ascertain the direction of effect, particularly when there are multiple complex linkages and when the impact of linkage errors work in opposite direction.	Detailed information about the characteristics of mothers more or less likely to link (subgroup-specific linkage rates), in order to identify groups that might be most affected by linkage error.
Measurement bias	Usual care for mothers not enrolled was not captured; some outcomes were measured by different professionals depending on whether the mother was enrolled in the intervention or not.	Bias in either direction. FNP nurses may have been morelikely to record positive outcomes if they have built a stronger relationship with enrolled mothers, but might also have been more likely to pick up on areas of need (ascertainment/surveillance bias).	Improved, high-quality data on community health contacts are needed at the individual level (including e.g. public health or adolescent pregnancy midwife services, average number of health visitor contacts, number of children’s centres).

*Missed links occur when a mother in the FNP Information System data is not linked to her health/education record and therefore appears twice in the data – once as an FNP mother with no linked health/education data, and once in the health data as being a mother who was not enrolled in the FNP; false links are likely to be less common, and occur when an FNP record is linked to the wrong health/education record, causing a mother not enrolled in the FNP to appear as though she was enrolled.