Historically, online response rates have been significantly lower than in-class response rates. The gap in response rates decreased as class size increases, but at best online response rate is 15% worse than in-class. For a large-N review, see IDEA Research Notes #4.
Nissen et al. (2018) found that online response rate could match in-class response rate for research based assessments when sufficiently motivated. In practice, to get high response rates for online administration of SETs, instructors should: send multiple email reminders, give multiple in-class reminders, and provide participation credit for completion. It is important to note that online response rates were lower (though not statistically significantly lower) for students who had lower course grades as compared to in-class response rates.