In order to lower costs, the idea has been raised to use a mixed-mode design for the UK Household Longitudinal Study (UKHLS) using Computer-Assisted Personal Interviewing (CAPI) and Web Self-Interviewing (WSI). However, a mixed-mode CAPI-WSI design may damage data-quality because of measurement effects between both modes. Before a decision is made about moving to a mixed-mode design in the UKHLS, measurement effects should be researched in greater detail by identifying questions sensitive to measurement effects, investigating survey design optimization for avoiding measurement effects, and investigating estimation techniques for measurement effects.