Response rates and patient retention are very important in long-term studies such as these, which used results from patient surveys as primary endpoints.
Both studies state in methods that they mailed surveys to patients and then reminded them to fill them out if necessary. The study by Ghogawala contacted patients up to 3 times (by phone? email?), producing a very good response rate of 68% at 4 years. However, Försth et al achieved an extremely high response rate of 96% at 5 years. How were these patients tracked and contacted?
Would the authors be able to comment any further on their methods for improving patient retention? Were patients incentivized in any way? Might the 96% response rate in the SSSS be a result of the Swespine registry or due to a difference in medical systems between the two countries?
I'm wondering if we can gain some insight from the authors and experts on maximizing response rates since we rely on patient-provided data for important studies like these.