Qu'est-ce que « Huang–Stoll model » ?

The Huang–Stoll model (basic form) writes the change of the transaction price between trades as

where $S$ is the traded spread and $\lambda$ the share of the half-spread by which the quotes move after a trade: adverse selection plus inventory, which the basic form cannot separate. Its extensions separate them with a model of the autocorrelation of trade signs.