Median of marine litter items by beach and marine region

• Not applicable

A subset of the Marine Litter Watch (MLW) dataset was selected according to the following conditions:
- events that took place between 2013 and 2020;
- events at one of the four European seas;
- events at sea beaches (since the MLW dataset also covers some river and lake shores);
- beach sites between 90 and 850 meters in length;
- data records that pass the quality control criteria; e.g. checking for duplication, location, litter count (zero or blank excluded, as well as paraffin/wax), or other data inconsistencies.
Such selection criteria yielded data on > 800 beaches in 27 countries covering the four regional seas of Europe – with one-third of all sites being located in three countries: France, Italy, and Spain. From these beaches, 1440 events were recorded, with the majority in 2015–2019. The reference dataset analysed includes 1,178,143 litter items. The number of events varies between marine regions. To compare the beach litter abundance across the events, the counts of litter items are normalised to transects of 100 metres.

The EU Technical Group on Marine Litter (TGML), the main policy advising body on marine litter, agreed to use the median as the averaging method to mean beach litter data among surveys. The median is the middle value when a dataset is sorted from smallest to greatest. Unlike the mean, very high or very low values do not affect the median value in the dataset. Consequently, when some of the values are extreme, their effect on the median is smaller and hence better than mean values.

Information is available in ETC/ICM report 1/2021: Marine litter and European beaches: learning from citizen science (see https://www.eionet.europa.eu/etcs/etc-icm/products/etc-icm-reports/etc-icm-report-1-2021-marine-litter-and-european-beaches-learning-from-citizen-science).