由于实际限制和进行大型临床试验所需的时间，观察性研究经常用于评估不同结直肠癌（CRC）筛查方法的比较有效性。然而，随时间变化的混杂因素，例如最后一次筛查中息肉的检测可能会导致统计结果出现偏差。最近，广义方法或 G 方法已被用于分析 CRC 筛查的观察性研究，因为它们能够解释这种随时间变化的混杂因素。当以连续尺度评估治疗和结果时，G 方法需要离散化或将连续函数转换为离散对应函数的过程。本文评估了时变混杂与离散化之间的相互作用，这可能会导致评估偏差筛选有效性。我们在评估典型筛查频率互不相同的不同 CRC 筛查方法的效果时研究了这种偏差。首先，利用理论，我们确定了偏差的方向。然后，我们使用假设设置的模拟来研究不同离散化水平、筛选频率和研究周期长度的偏差大小。我们开发了一种方法来评估模拟情况下因粗化而可能出现的偏差。所提出的方法可以通过确定数据离散化间隔长度的选择来为未来的筛查有效性研究提供信息，特别是对于 CRC，以最大限度地减少粗化造成的偏差，同时平衡计算费用。© 作者 2024；版权所有。由牛津大学出版社代表国际流行病学协会出版。

Observational studies are frequently used to estimate the comparative effectiveness of different colorectal cancer (CRC) screening methods due to the practical limitations and time needed to conduct large clinical trials. However, time-varying confounders, e.g. polyp detection in the last screening, can bias statistical results. Recently, generalized methods, or G-methods, have been used for the analysis of observational studies of CRC screening, given their ability to account for such time-varying confounders. Discretization, or the process of converting continuous functions into discrete counterparts, is required for G-methods when the treatment and outcomes are assessed at a continuous scale.This paper evaluates the interplay between time-varying confounding and discretization, which can induce bias in assessing screening effectiveness. We investigate this bias in evaluating the effect of different CRC screening methods that differ from each other in typical screening frequency.First, using theory, we establish the direction of the bias. Then, we use simulations of hypothetical settings to study the bias magnitude for varying levels of discretization, frequency of screening and length of the study period. We develop a method to assess possible bias due to coarsening in simulated situations.The proposed method can inform future studies of screening effectiveness, especially for CRC, by determining the choice of interval lengths where data are discretized to minimize bias due to coarsening while balancing computational costs.© The Author(s) 2024; all rights reserved. Published by Oxford University Press on behalf of the International Epidemiological Association.