Demographic data, such as highest level of education attained, are often assumed to be relatively free from measurement error. As part of an evaluation of an early childhood parenting intervention, 654 parents reported their highest level of education via telephone interview at baseline and self-directed questionnaire at follow-up 5.7 years later. At follow-up, 14% reported a lower level of education compared to baseline, indicating measurement error in one of the data collections. Comparison with data collected by an external agency for a subsample of participants (n = 261) 3.2 years after baseline indicated error in both the baseline and follow-up data. Probable causes of error included respondent and interviewer confusion regarding the names of post-school qualifications and item construction incorrectly implying linear pathways through education. We make recommendations around question construction and data collection methods for reducing measurement error in self-reported educational attainment.