Bertil,
I'm certainly no expert on trending or filters, but I've read a little about this concept. Most applications that I've seen separate a pool of numbers into groups. Those groups are then cross-referenced with temporal draw ranges. I suppose the groups and ranges can be arbitrarily set by the person employing such a method, or they might be built into the software which incorporates them. For that matter, every number in a pool could be analyzed individually rather than as a part of a group.
At any rate, the actual occurrence of any given number or group is compared to the expected average occurrence in each draw range. As the percentage of occurrence of a number or group of numbers changes from one draw range to the next, it is said to be "trending." The percentage of change from one draw range to the next is presumably indicative of the likelihood that a number or group of numbers will occur in future drawings. Larger and more successive gains would indicate strength whereas smaller, sporadic or negative changes would indicate decreased likelihood.
This all seems like a bit much to do manually from draw to draw, but I suppose it could be done. If you're interested in giving it a try, you might shop around for some software that incorporates this type of filter. I have no idea what software that might be, but you might ask some of the other members. From what I read about filters in the posts, BobP, RJOh and WinD all seem to be fairly well versed on the subject. Maybe one of them can shed a bit more light on this subject. By no means should you take my rather vague understanding of this subject as gospel. I'd certainly like to hear from some more knowledgeable people on this subject.
Thanks for raising a very good topic. I hope it will draw some thoughtful responses.
Good luck,
aye'