
https://www.auswaertiges-amt.de/blob/2405080/23e76da338f1a1c06b1306c8f5f74615/201013-nap-monitoring-abschlussbericht-data.pdf

https://webunwto.s3.eu-west-1.amazonaws.com/s3fs-public/2020-07/200728-barometer-en.pdf

https://www.boeckler.de/pdf/p_study_hbs_376.pdf