## How do you assess loss to follow-up?

To assess the potential impact of loss to follow-up on the estimates of treatment effect we will, for varying assumptions about the outcomes of participants lost to follow-up (LTFU), calculate (1) the percentage of RCTs that lose statistical significance and (2) the mean change in effect estimate across RCTs.

## Is there loss to follow-up in experimental studies?

Many would consider the loss to follow-up rate to be 9 (18%) of 49 in treatment A and 11 (21%) of 52 in treatment B using as the denominator only those that were treated. However, the real proportion lost to follow-up must consider those who were randomly assigned, even if they did not receive treatment.

**Can people lost to follow-up be included in analysis?**

One way to complete statistical analysis where participants are lost to follow-up could be to exclude those lost from the trial itself, such that all the results of the remaining participants may still be analysed. Hence, only those who followed the protocol are included in the trial.

**How does loss to follow-up affect validity?**

LOSS TO FOLLOW-UP AND TYPES OF MISSING DATA If follow-up is incomplete or interrupted, leading to missing data at the end of the study, this could impact the internal validity of the study.

### What causes loss to follow-up?

Common reasons for loss to follow-up were social or structural. These included problems with transportation, finances, and work/child care responsibilities. Among those lost to follow-up, subsequent outcomes were heterogeneous.

### Is loss to follow-up information bias?

A loss-to-follow-up bias occurs in prospective cohort studies. With this type of bias, the true relationship between exposure and disease will only be distorted if the losses during follow-up are selective (non-random) with respect to both exposure and outcome.

**Why does loss of follow up occur?**

Losses to follow-up can introduce bias (a deviation of the observed value of the measure of association from the value that would have been observed in the absence of bias) if there are differences in likelihood of loss to follow-up that are related to exposure status and outcome.

**How does loss to follow-up affect study results?**

## What type of bias is loss to follow-up?

Selection bias due to loss to follow up is the absolute or relative bias that arises from how participants are selected out of a given risk set 3.

## Why is loss to follow-up a problem?

Loss to follow up is a problem for two main reasons: It reduces the effective sample size because the investigators will be missing outcome measures on those who are lost. If follow up rates differ among comparison groups and if attrition is related to the outcome, the results of the study can be biased.

**Why is follow up important in clinical trials?**

Prolonged follow-up of trial participants after the end of the scheduled trial period can provide important information on both efficacy and safety outcomes. This protocol describes a systematic review to qualitatively compare methods of post-trial follow-up used in large randomized controlled trials.

**Why are patients lost to follow-up?**