Some socialists such as H. D. Dickinson and Oskar Lange, responded by invoking general equilibrium theory, which they argued disproved Mises's thesis. They noted that the difference between a planned and a free market system lay in who was responsible for solving the equations. They argued, if some of the prices chosen by socialist managers were wrong, gluts or shortages would appear, signalling them to adjust the prices up or down, just as in a free market. Through such a trial and error, a socialist economy could mimic the efficiency of a free market system, while avoiding its many problems.