when participants are dissatisfied with the conduct of the trial but have not yet withdrawn, the investigator should make an effort to address their concerns and retain them in the trial, rather than simply indicating that the participants withdrew consent. In doing so, investigators must be careful that their efforts do not cross over into coercion.

Fourth, designers and managers need to consider how investigators are paid. Paying investigators solely by the number of participants enrolled should be avoided because it places too much emphasis on enrollment and not enough on follow-up; payments should also reflect follow-up work (e.g., payment per visit or procedure). In addition, linking some additional compensation to the completeness of the data collection should be considered. It is acceptable and generally advisable to link a final payment to completion of forms at a study closeout visit (i.e., a final visit at the end of follow-up to assess the participant’s status). However, care must be taken on this point. Providing extra compensation to investigators for encouraging participants to complete a study when the participants are thereby exposed to significant additional risks could create a conflict of interest on the part of the investigator and would therefore be unethical. But if there are minimal risks associated with data collection to the participant, it may be acceptable to provide financial incentives to the investigator to continue to collect data, whether or not the participant continues treatment.

Fifth, designers and managers can ensure that data collection is monitored and reported during the trial. Missing data and missed visits that could affect important outcomes need to be assessed in real time by site personnel during a clinical trial. The information from these assessments should be available and shown to investigators at regular meetings and on study websites, creating a climate to encourage other investigators to collect more complete data. Also, identification of poorly performing sites can help identify the need for some sort of remediation, including additional training, site visits, or even site closure. Site visits should be targeted on the basis of assessments of the amount of missing data, with the goal of helping to correct the problem.


Investigators and site personnel can also act in several ways to reduce the amount of missing data. First, in the informed consent process, they can emphasize to participants the importance of continued participation for the full duration of the trial. Similarly, they can ensure that the trial procedures allow for an informed withdrawal of consent so that participants recognize the importance of continued follow-up for data collection if they discontinue study treatment: see Box 3-1 for an example of language for withdrawal. Second, investigators and site personnel can provide incentives

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