Essential information > Understanding UK Biobank > Repeat assessment (2012-13)

Repeat assessment (2012-13)

A repeat assessment of 20,000 participants was carried out between August 2012 and June 2013 at the UK Biobank Co-ordinating Centre, Stockport, UK. Participants who lived within a 35 km radius of the assessment centre were invited via email or letter, with an overall response rate of 21%. (See Field 110008 for invitation dates and Field 110003 for responses to invitation.)

Participants underwent a repeat of the baseline assessment visit, which included obtaining information on a participant’s health and lifestyle, hearing and cognitive function, collected through a touchscreen questionnaire and brief verbal interview. A range of physical measurements were also performed, which included: blood pressure; arterial stiffness; eye measures (visual acuity, refractometry, intraocular pressure, optical coherence tomography); body composition measures (including impedance); hand-grip strength; ultrasound bone densitometry; spirometry; and an exercise/fitness test with ECG. Samples of blood, urine and saliva were also collected.

The repeat assessment data collected not only enriches the UK Biobank resource with information on participant health and lifestyle over time, but also provides an opportunity to calibrate existing measures and account for regression dilution bias.

Regression dilution bias

Variation in the measurements captured at baseline may lead to underestimated associations between risk factor and outcome – known as regression dilution bias. Regression dilution bias can be introduced through a variety of different ways, including, through measurement error, through short-term biological variability (e.g. diurnal or seasonal variation) or through longer-term within-person variability (e.g. changes to diet and physical activity or medication use, which could modify risk factor values at the time of measurement). The information collected through re-assessment of a large subset of participants can be used to adjust for regression dilution bias and produce more precise measures for the entire cohort.

Study responders

The characteristics of those who participated in the repeat assessment versus those who declined invitation or were non-responders are provided in the report: Characteristics of responders and non-responders.