Group together with another pair of students. Share the items that you included on your timeline. Share the evidence in the article(s) that prompted you to write those items. If your team and the new team of students agrees, compare with another group of students. If you disagree with the other pair of students, have an educational argument to determine which pair has more evidence to support the item being on the timeline. If both groups have proper evidence, maybe both items belong on the timeline!