Missing income data in the millennium cohort study: Evidence from the first two sweeps Authors: Denise Hawkes and Ian Plewis Discussant: Nicholas Biddle

Introduction and overview  Data – Millennium Cohort Study  Research questions – What are the factors associated with non-response? More specifically:  Are there within household and individual correlations for missing income data?  Is the sex of the interviewer an important explanatory variable?  How is missing data in sweep one related to missing data in sweep two?  Is attrition at sweep two related to the level of household income or the failure to provide data in sweep one?  Method –  Descriptive analysis  Binary and Multinomial Logit models with non-response as dependent variable  Binary Logit with attrition between sweep one and sweep two as dependent variable

Data  Millennium Cohort Study  First sweep – 18,819 babies born in the UK from 1 st September 2000 (from 18,552 families). Interviewed when baby was 9 months old  Second Sweep – 14,898 families from original sample and 692 new families. Interviewed when children around 3 years old.  Information from main respondent (usually mother) and partner of respondent (usually father)  Incomplete information on income through:  Unit non-response (response rate 72% in first sweep)  Partner non-response (88% of families with partners responded)  Item non-response for income (6% of main respondents and partners did not provide income data)  Attrition between sweeps (79% of eligible families responded in sweep two)  Income information:  Collected from those currently doing paid work, those who have a paid job but are on leave, those who have worked in the past but have no current job.  For employees – total take home pay and gross pay  For self employed – ‘amount you personally took out of the business after all taxes and costs’

Data  Millennium Cohort Study  First sweep – 18,819 babies born in the UK from 1 st September 2000 (from 18,552 families). Interviewed when baby was 9 months old  Second Sweep – 14,898 families from original sample and 692 new families. Interviewed when children around 3 years old.  Information from main respondent (usually mother) and partner of respondent (usually father)  Incomplete information on income through:  Unit non-response (response rate 72% in first sweep)  Partner non-response (88% of families with partners responded)  Item non-response for income (6% of main respondents and partners did not provide income data)  Attrition between sweeps (79% of eligible families responded in sweep two)  Income information:  Collected from those currently doing paid work, those who have a paid job but are on leave, those who have worked in the past but have no current job.  For employees – total take home pay and gross pay  For self employed – ‘amount you personally took out of the business after all taxes and costs’

Data  Millennium Cohort Study  First sweep – 18,819 babies born in the UK from 1 st September 2000 (from 18,552 families). Interviewed when baby was 9 months old  Second Sweep – 14,898 families from original sample and 692 new families. Interviewed when children around 3 years old.  Information from main respondent (usually mother) and partner of respondent (usually father)  Incomplete information on income through:  Unit non-response (response rate 72% in first sweep)  Partner non-response (88% of families with partners responded)  Item non-response for income (6% of main respondents and partners did not provide income data)  Attrition between sweeps (79% of eligible families responded in sweep two)  Income information:  Collected from those currently doing paid work, those who have a paid job but are on leave, those who have worked in the past but have no current job.  For employees – total take home pay and gross pay  For self employed – ‘amount you personally took out of the business after all taxes and costs’

Patterns of income response  Original sample (paper has information on new families and proxies) Sweep oneSweep two MainPartnerMainPartner Income response45.9%64.7%50.6%62.9% Don’t know1.8%2.1% Refusal0.9%2.1% Total non-response2.7%4.3%4.4%8.7% Not applicable51.5%31.0%45.1%28.4% Sample18,55214,898

Patterns of income response  Original sample (paper has information on new families and proxies) Sweep oneSweep two MainPartnerMainPartner Income response45.9%64.7%50.6%62.9% Don’t know1.8%2.1% Refusal0.9%2.1% Total non-response2.7%4.3%4.4%8.7% Not applicable51.5%31.0%45.1%28.4% Sample18,55214,898

Partner and main respondent income response – Sweep one Partner respondent Don’t know/ refusal Not applicable Income response Don’t know/refusal (464) 26.6% 27.4%45.9% Main respondent Not applicable (10,264) 3.9% 42.7% 53.4% Income response (7,824) 3.5%18.0% 78.5%

Partner and main respondent income response – Sweep two Partner respondent Don’t know/ refusal Not applicable Income response Don’t know/refusal (614) 26.7% 29.0%44.3% Main respondent Not applicable (7,190) 9.6% 36.5% 54.0% Income response (7,094) 6.5%21.0% 72.5%

Sweep one and sweep two income response – Main respondent Sweep two Don’t know/ refusal Not applicable Income response Don’t know/refusal (357) 17.9% 26.7%55.4% Sweep one Not applicable (7,733) 2.9% 74.4% 22.8% Income response (6,504) 5.3%14.8% 79.9%

Sweep one and sweep two income response – Partner Sweep two Don’t know/ refusal Not applicable Income response Don’t know/refusal (501) 35.2% 0.4%64.4% Sweep one Not applicable (1,778) 22.9% 2.4% 74.7% Income response (7,433) 8.7%0.1% 91.2%

Modelling non-response – Main respondent Sweep oneSweep two Spec. (I)Spec. (II)Spec. (III) Self employed Has a partner Social classIntermediate1.6 - Reference managerialSmall employers and self employment1.8 and professionalLower supervisors and technical Semi routine and routine EthnicityMixed - Reference whiteIndian Pakistani and Bangladeshi Black or Black British1.6 Other ethnic group2.3 CountryWales - Reference EnglandScotland Northern Ireland Respondent did not respond in sweep one--3.0 Respondent same in sweep one and two Sample Size8,1905,800

Modelling non-response – Main respondent Sweep oneSweep two Spec. (I)Spec. (II)Spec. (III) Self employed Has a partner Social classIntermediate1.6 - Reference managerialSmall employers and self employment1.8 and professionalLower supervisors and technical Semi routine and routine EthnicityMixed - Reference whiteIndian Pakistani and Bangladeshi Black or Black British1.6 Other ethnic group2.3 CountryWales - Reference EnglandScotland Northern Ireland Respondent did not respond in sweep one--3.0 Respondent same in sweep one and two Sample Size8,1905,800

Modelling non-response – Partner (I) Sweep oneSweep two Spec. (I)Spec. (II)Spec. (III) Self employed Social classIntermediate - Reference managerialSmall employers and self employment3.0 and professionalLower supervisors and technical0.68 Semi routine and routine0.66 NVQ Level 1 NVQ LevelsNVQ Level Reference noneNVQ Level NVQ Level NVQ Level Other/overseas qual only EthnicityMixed Reference whiteIndian Pakistani and Bangladeshi Black or Black British Other ethnic group2.0 Owner occupier

Modelling non-response – Partner (I) Sweep oneSweep two Spec. (I)Spec. (II)Spec. (III) Self employed Social classIntermediate - Reference managerialSmall employers and self employment3.0 and professionalLower supervisors and technical0.68 Semi routine and routine0.66 NVQ Level 1 NVQ LevelsNVQ Level Reference noneNVQ Level NVQ Level NVQ Level Other/overseas qual only EthnicityMixed Reference whiteIndian Pakistani and Bangladeshi Black or Black British Other ethnic group2.0 Owner occupier

Modelling non-response – Partner (II) Sweep oneSweep two Spec. (I)Spec. (II)Spec. (III) CountryWales - Reference EnglandScotland Northern Ireland Respondent did not respond in sweep one Respondent same in sweep one and two Sample Size10,7547,893

Other modeling – Multinomial Logit and attrition  Multinomial Logit – Response vs. don’t know vs. refuse  Main respondent:  Self employed only significantly more likely to be ‘don’t know’ not ‘refusal’  Same with social class variables  Black or Black British as well as Northern Ireland more likely to refuse  Partner respondent:  Self employed significantly more likely to refuse and not know  NVQ levels and ethnicity both associated with refusal  Attrition at sweep two  Higher income in sweep one associated with lower odds of attrition between sweep one and sweep two  Main income and partner income non-response in sweep one associated with higher odds of attrition between sweep one and sweep two

Other modeling – Multinomial Logit and attrition  Multinomial Logit – Response vs. don’t know vs. refuse  Main respondent:  Self employed only significantly more likely to be ‘don’t know’ not ‘refusal’  Same with social class variables  Black or Black British as well as Northern Ireland more likely to refuse  Partner respondent:  Self employed significantly more likely to refuse and not know  NVQ levels and ethnicity both associated with refusal  Attrition at sweep two  Higher income in sweep one associated with lower odds of attrition between sweep one and sweep two  Main income and partner income non-response in sweep one associated with higher odds of attrition between sweep one and sweep two

Summary  Household and individual correlations for missing income data  Self employment, some ethnic groups (though not consistent), Northern Ireland  The sex of the interviewer is not an important explanatory variable in explaining income non-response  Some variables only associated with ‘don’t know’ or ‘refusal’ only  Missing data in sweep one associated with higher odds of missing data in sweep two  Especially amongst partner respondents  Higher household income in sweep one associated with lower attrition in sweep two  Missing data in sweep one associated with higher attrition in sweep two

Suggested further work and information  Models for non-response  More diagnostic information (e.g. tests of group significance)  Information on the child?  Interviewer bias  Multilevel model?  Interactions or other information on the interviewer  Implications for survey design  Difference between don’t know and refusal