You use ibid for a reference entry when the citation is the same as the previous footnote or endnote. If the page number is different, you include the page number of the new entry after ibid. ibid saves you writing out the full reference again; for example, Y. Anzai and H. A. Simon. The theory of learning by doing. Psychological Review, 86, 124-180, 1979, p. 126; ibid. ibid., p.157. op.cit. is.