1. https://itsalljustcomics.com/all-time-record-comic-book-sales, (2018).
2. https://www.bbc.com/news/entertainment-arts-28936096, (2018).
3. https://www.huffingtonpost.com/2014/08/27/superman-action-comics-no_n_5719399.html?guccounter=1, (2018).
4. https://www.cbcscomics.com/grading, (2018).
5. https://www.cgccomics.com/grading, (2018).