Answer:
Attrition bias
Explanation:
When conducting an experiment, it can happens that certain participants drop out before the experiment/study comes to an end. When this happens, we say that the study might have attrition bias.
This bias is also known as sample attrition and it may affect the external and the internal validity of a trial.
In this example, 6 people dropped out before the last session of the study, therefore, we can say that this study might have attrition bias.