Mixed modes are becoming increasingly popular in surveys. This approach can decrease costs and non-response bias. But in order to evaluate the utility of this approach we must separate selection and measurement effects of the different modes. In this paper I propose a new way of applying the front-door method to control for measurement differences between modes: equivalence testing with latent measurement models. A small simulation study will show how this approach works and how it can be biased if the assumptions of exhaustiveness and isolation are not true in the observed data.