For the 2015 year end data, Diamond provided the top 1,000 comics and the top 1,000 trade lists, plus a few items in both categories from smaller publishers which fell under the top 1,000 lists. Neither the top 1,000 comics nor the top 1,000 trades lists included index values for the items. I took the total known sales for each issue over the year and used that for the estimated sales figure.
The estimated units bounce around by the number due to sales which fell below the radar of the top comics and trades list each month. For example, "Spider-Gwen (2015)" #1 at rank 8 had to have at least 939 unreported units in order to outsell "Invincible Iron Man" #1 which ranked right below it. However, since "Spider-Gwen (2015b)" #1 at rank 22 had reported sales exceeding the item below it, it didn't need any unreported units to place where it did on the list. Some items never appeared on the monthly top sellers lists and therefore have no reported sales. For example, "Invincible" #118 was not included in the March 2015 data because of the promotion $0.25 price point and as a result has no reported sales. Based on it being at rank 430, it had to sell at least 46,197 units since that is how well "Patsy Walker AKA Hellcat" #1 sold which is right below it on the list.
Marvel had 517 of the items list of the top 1,000 comics for 2015 totaling to 35,285,403 units. DC had 381 items with a total of 18,213,467 units. Image was a distant third with 62 items totaling to 2,969,020 units The other publishers in the top 1,000 list were Dark Horse, IDW Publishing, Oni Press, Archie Comics, Valiant, Titan Comics and Dynamite Entertainment. On the trades list, DC took 341 of the top 1,000 slots, Marvel had 287 item and Image had 166 items on the list. Dark Horse had 60 items on the list, IDW had 35 and the remaining 111 items on the list were from another two dozen publishers.