This study examines the two-year follow-up (data collected in 2016 at modal age 21/22) of an original mixed-mode longitudinal survey experiment (data collected at modal age 19/20 in 2014). The study compares participant retention in the experimental conditions to retention in the standard Monitoring the Future (MTF) control condition (participants who completed an in-school baseline survey in 12th grade in 2012 or 2013 and were selected to participate in the first follow-up survey by mail in 2014, _N_=2,451). A supplementary sample who completed the 12th grade baseline survey in 2012 or 2013 but were _not_ selected to participate in the main MTF follow-up (_N_=4,950) were recruited and randomly assigned to one of three experimental conditions in 2014 and again in 2016: 1: Mail Push, 2: Web Push, 3: Web Push + Email. Results from the first experiment indicated that Condition 3 (Web Push + Email) was promising based on similar response rates and lower costs. The current study examines the associations of experimental condition and type of response in 2014 with participation in 2016, the extent to which response mode and device type changed from 2014 to 2016, and cumulative cost comparisons across conditions. Results indicated that responding via web in 2014 was associated with greater odds of participation again in 2016 regardless of condition; respondents tended to respond in the same mode although the “push” condition did move respondents toward web over paper; device type varied between waves; and the cumulative cost savings of Web Push + Email grew larger compared to the MTF Control. The web push strategy is therefore promising for maintaining respondent engagement while reducing cost.