During the structured interview as part of the cognitive testing appointment at age 70 , respondents provided the information required for all the covariates except for father 's social class , which was obtained via a questionnaire , and BMI , which was obtained during the physical examination . Covariates were selected a priori based on previous literature ( Deary et al . , 2009 ) , including sex , Occupational Social Class ( OSC ) ( Office of Population Censuses and Surveys , 1980 ) of participant 's father , people per room in childhood household , childhood smoking status , OSC of participant , smoking status and alcohol consumption . OSC of the participant 's father was used as a measure of socioeconomic status from childhood and dichotomised into Professional - managerial ( I and II ) and Skilled , partly skilled , unskilled ( III , IV and V ) . Additionally , we used childhood overcrowding as a secondary measure of socioeconomic status , defined as the number of people per household room . Participants were asked at what age they started smoking and were categorised as a childhood smoker if they responded that they had started before age 16 , as per convention ( Hopkinson et al . , 2014 ) . Smoking status at age 70 was dichotomised as smoker or non - smoker . Similarly , alcohol consumption was dichotomised as drinks alcohol or does n't drink alcohol . BMI at age 70 was calculated as weight divided by height squared . Socioeconomic status during adulthood was defined by OSC of the participant 's main job from their career and dichotomised in the same way as their father 's OSC . For females , husband 's OSC was used if higher than their own . We selected a number of variables as effect modifiers a priori including sex , adulthood OSC and APOE e4 allele . Sex and adulthood OSC at age 70 were operationalised as above . Blood samples were taken during examination and participants were genotyped for APOE allele status using TaqMan technology at the Wellcome Trust Clinical Research Facility Genetics Core ( Deary et al . , 2012a ) . Participants were dichotomised into having at least one APOE e4 allele or having none . In addition to the variables above , we selected several variables to act as auxiliary variables in the imputation of missing data . Due to the association with cognitive ageing , these included the Hospital Anxiety and Depression Scale score ( Zigmond and Snaith , 1983 ) , history of stroke ( Yes or No ) and family history of heart disease , stroke , or problems with blood vessels ( Yes or No ) .