Triangulation is the strategy of integrating the findings from multiple different methods to get more reliable answers to research questions. Each method has its sources of possible bias, which are independent of each other. When it comes to identifying causal relationships in epidemiology, if the results of different methods all lead to the same conclusion, this increases confidence in the findings.
References:
Lawlor, Debbie A et al. Int J Epidemiol. vol. 45,6 (2016)