The present work deals with an application of statistics to different choices of investment in advertising. The distribution of market-shares of two telephone guide companies (G_A and G_B) has been analyzed and in particular the choices of investment made by lots of companies from 1997 to 2001 have been taken into account. Starting from the time series of their investments in G_A and G_B, we have tried to identify the probability for the companies to join the competitive group. The aim of the research is reached through the use of Markov chains to discover the various types of investing behaviour in the long period. Finally, through a stratification and a separate estimate of the transition probabilities, the main differences of market-orientation for the two advertising channels are discussed.