We present a methodology to quantify the effects of behavior change techniques (BCTs) that allows forecasting campaign effects on behavior and psychological constructs. The approach involves the gathering of longitudinal data during actual campaigns in which different combinations and sequences of BCTs are applied to different groups. Approximate metric data are gathered by asking for simple and specific evaluations. The data are analyzed using regression models that consider the value range of the dependent variable as bounded (bounded linear regression). Based on these models, forecasts of the intervention effects are calculated, considering the uncertainty of the parameter estimates. The methodology is applied to investigate the effects of prompts (external memory aids), public self-commitments, and implementation intentions on affective and instrumental attitudes, injunctive and descriptive norms, forgetting, perceived behavior control, and behavior in a health-promotion campaign in Bolivia. Prompts and public self-commitments reached more than half of the target population but only showed relevant effects when combined or repeated. The effects of both BCTs on behavior were mainly mediated by forgetting. Implementation intentions were not well received by the promoters and the population. From the few cases that implemented this BCT, no clear psychological effects could be derived.