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Preventing and Treating Missing Data in Longitudinal Clinical Trials by Craig Mallinckrodt

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2 Missing Data Mechanisms

2.1 Introduction

One of the keys to understanding the potential impact of missing data is to understand the mechanism(s) that gave rise to the missingness. However, before considering missing data mechanisms, two important points are relevant. First, there is no single definition of a missing value. Even if restricting focus to dropout (withdrawal), several possibilities exist. For example, values may be missing as the result of a patient being lost to follow-up, with nothing known about treatment or measurements past the point of dropout. Alternatively, a patient may withdraw from the initially randomized study medication and be given an alternative (rescue) treatment, but with no further measurements taken. Or, follow-up ...

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