Less active vessels (vessels that earn less than £10k per year) were removed from the dataset to reduce activity bias on the estimates.
Cafodd cychod llai gweithredol (cychod sy'n ennill llai na £10 mil y flwyddyn) eu tynnu o'r set ddata i leihau rhagfarn gweithgarwch ar yr amcangyfrifon.