MITT estimator
’ initiation status, ’
EVENT
The mITT estimator, which excludes participants who do not begin treatment from the analysis population, can be written as
where To evaluate the properties of this estimator, we begin by outlining the four principal strata defined by the intercurrent event ‘failure to initiate treatment’ (The four principal strata defined by the intercurrent event ‘failure to initiate treatment’.mITT: modified intention-to-treat.The mITT estimator excluding participants who do not initiate treatment compares the shaded cells in the intervention column against the shaded cells in the control column.The ‘always initiators’ are participants who would initiate treatment in either treatment group (i.e. for whom In an RCT, randomisation can be seen to balance the treatment groups within each stratum in expectation. Hence, if we knew the principal stratum membership of each participant, we could perform the analysis in the stratum of interest and obtain an unbiased estimator based on a randomised comparison. However, in practice, we do not know the principal stratum membership, as this requires knowledge of participants’ initiation status under both treatment conditions, but we can only observe a participant’s initiation status under the treatment arm to which they were randomised. For instance, if a participant allocated to the intervention initiated treatment, we have no way of telling whether they are an ‘always initiator’ or an ‘intervention initiator’.We can see from 