sentences:  Methods like sequential subject matching, background timing and mouse use tracking, and instantaneous compensation through PayPal have the potential to address many of the concerns about the internal validity of web-based experiments.These methods control for differences in response times, address issues of selective attrition, concentration, and distraction, minimize subject concern about compensation, improve subject confidence that they have a real human partner in the experiment, and ensure that subjects have an appropriate understanding of the instructions and the decision problems in the experiment.Scholars have also formulated techniques to decrease or account for drop-outs, including the high-hurdle technique (motivationally adverse information is clustered at the beginning of the study), the seriousness check (requesting participant's probability estimate that they'll complete the study), and the warm-up phase (placing consent forms or other pre-study materials first to winnow the samples before the study begins).