RCT - Justify assessment of impact that appears over a longer period of time

How can we justify assessment of impact that appears over a longer period of time especially when chances of losing participants to follow up is high?
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Throughout 2015, BetterEvaluation partnered with the UNICEF Office of Research - Innocenti to develop eight impact evaluation webinars for UNICEF staff. The objective was to provide an interactive capacity-building experience, customized to focus on UNICEF’s work and the unique circumstances of conducting impact evaluations of programs and policies in international development. The webinars were based on the Impact Evaluation Series - a user-friendly package of 13 methodological briefs and four animated videos - and presented by the briefs' authors. You can view the full series here: betterevaluatio...
This page provides links not only to the eight webinars, but also to the practical questions and their answers which followed each webinar presentation.*
The findings, interpretations and opinions expressed in the webinars are those of the presenters and do not necessarily reflect the policies or views of the United Nations Children’s Fund (UNICEF). The presenters are independent impact evaluation experts who were commissioned by UNICEF to prepare the webinars and use their own knowledge and judgement on key issues and to provide advice. The questions and comments reflected in the Q & A materials are based on those submitted by UNICEF staff as part of this capacity-building initiative. They do not necessarily reflect the policies or views of UNICEF.
The webinars were commissioned by UNICEF and UNICEF is entitled to all intellectual property and other proprietary rights which bear a direct relation to the contract under which this work was produced. The materials on this page are subject to a Creative Commons license CC BY-NC (Attribution-NonCommercial) and may be used and reproduced in line with the conditions of this licence.