NCDS response and missingness

NCDS response

The table below presents statistics about response in NCDS at every major sweep from birth.

Of the 17,415 cohort members who participated in the first sweep, 4,497 (25.8%) have participated in all 11 major sweeps.

Of all 18,558 cohort members, 12,505 (67.4%) have taken part in at least half the sweeps (six or more sweeps).

Table: Participation in NCDS from birth to 55 years

^a The original sample was supplemented by migrants born in 1958

Figure 1: NCDS response (as % of eligible sample) over time

Missingness in NCDS

NCDS variables

We have used a systematic data-driven approach to identify variables that are important predictors of non-response at each NCDS sweep, between age seven and age 55. Future work will extend this analysis to include non-response at the age 62 sweep.

A list of these variables is provided in the appendix of the Handling missing data in the CLS cohort studies user guide.

These variables can then be considered for inclusion in analyses – for example as auxiliary variable when using multiple imputation – in order to maximise the plausibility of the missing at random (MAR) assumption.

Read more about the process in the article: Missing at random assumption made more plausible: evidence from the 1958 British birth cohort.

Example: restoring the composition of NCDS at age 50 and 55

We have been able to restore the composition of the NCDS samples at ages 50 and 55 to be more representative of the study’s target population. We did this by including predictors of non-response at age 50 and age 55 as auxiliary variables in multiple imputation analyses.

For example, we were able to replicate:

• the original distribution of paternal social class observed at the birth survey
• the distribution of cognitive ability at age seven
• the known population distribution of educational attainment and marital status at age 50.

Figure 2: Social class of mother’s husband at birth before and after adjustment for missing data

The imputation phase of this analysis included predictors of non-response at age 55 and social class at birth only for cohort members that participated at age 55.

Figure 3: Percentage of those with degree or equivalent at age 50 in the Annual Population Survey and NCDS before and after adjustment for missing data.

APS GB: Annual Population Survey = Born in Great Britain in 1958, derived by the Office for National Statistics (ONS)

APS All: Annual Population Survey = Born in Great Britain or elsewhere in 1958 (derived by ONS)

NCDS50 MI: Estimate after multiple imputation using predictors of educational attainment at age 50 and predictors of non-response at age 50 as auxiliary variables.