According to two decades worth of research from Dr. H. Nejat Seyhun compiled in his book Investment Intelligence from Insider Trading, buying is most predictive when it (1) comes from the CEO or other top-level executive, and (2) it's performed in bulk. Seyhun found buys of between 10,000 and 100,000 shares to be most informative.
Dr. H. Nejat Seyhun wrote Investment Intelligence from Insider Trading in which he reported the result of a study covering 21 years of insider trading. In the August 2004 issue of Motley Fool Hidden Gems, Tom Gardner interviewed Dr. Seyhun.
Among other results, Dr. Seyhun noted the following: