From the glossary of the ICH E9: estimands addendum:
A precise description of the treatment effect reflecting the clinical question posed by the trial objective. It summarises at a population-level what the outcomes would be in the same patients under different treatment conditions being compared.
This entire page is a starting point. Further,
Aroda, V. R., T. Saugstrup, J. B. Buse, M. Donsmark, J. Zacho, and M. J. Davies. 2019. “Incorporating and interpreting regulatory guidance on estimands in diabetes clinical trials: The PIONEER 1 randomized clinical trial as an example.” Diabetes Obes Metab 21 (10): 2203–10.
Casey, Michelle, Evgeny Degtyarev, Maria Jose Lechuga, Paola Aimone, Alain Ravaud, Robert J. Motzer, Feng Liu, et al. 2021. “Estimand Framework: Are We Asking the Right Questions? A Case Study in the Solid Tumor Setting.” Pharmaceutical Statistics 20 (2): 324–34. https://doi.org/https://doi.org/10.1002/pst.2079.
Degtyarev, Evgeny, Yiyun Zhang, Kapildeb Sen, David Lebwohl, Mouna Akacha, Lisa V. Hampson, Bjoern Bornkamp, Antonella Maniero, Frank Bretz, and Emmanuel Zuber. 2019. “Estimands and the Patient Journey: Addressing the Right Question in Oncology Clinical Trials.” JCO Precision Oncology, no. 3: 1–10. https://doi.org/10.1200/PO.18.00381.
Lawrance, Rachael, Evgeny Degtyarev, Philip Griffiths, Peter Trask, Helen Lau, Denise D’Alessio, Ingolf Griebsch, Gudrun Wallenstein, Kim Cocks, and Kaspar Rufibach. 2020. “What Is an Estimand & How Does It Relate to Quantifying the Effect of Treatment on Patient-Reported Quality of Life Outcomes in Clinical Trials?” J Patient Rep Outcomes 4 (1): 68. https://doi.org/10.1186/s41687-020-00218-5.
Lynggaard, Helle, James Bell, Christian Lösch, Amel Besseghir, Khadija Rantell, Volker Schoder, and Vivian Lanius. 2022. “Principles and Recommendations for Incorporating Estimands into Clinical Study Protocol Templates.” Trials 23 (1): 685. https://doi.org/10.1186/s13063-022-06515-2.
Sun, S., J. Weber, E. Butler, K. Rufibach, and S. Roychoudhury. 2021. “Estimands in Hematology Trials.” Pharm. Stat., to Appear. https://arxiv.org/abs/2010.00957.