Preferred shares are often issued with terms that are virtually identical with another issue from the same issuer; in some other cases, the terms of two series may be different, but the series may be convertible into each other at specified times. Investors should look for this kind of pairing, because the market sometimes does not price these issues as similarly as one might expect.
Look for the research link!
Regular readers will know that I am currently enraged by the persistent difference in price between BAM.PR.M and BAM.PR.N!
Update, 2015-5-12: Note that I have also provided a link to the calculator.