This was not, however, the end of Keynesianism. Some economists felt that the traditional Keynesian model still offered some useful insights. One of the main criticisms of the Keynesian model is that it had no micro foundations. This led to the development of New Keynesianism. The central idea is that the argument of those who criticised Keynes depended upon the economy being able to adjust quickly and easily. There are many models which fit into the New Keynesian paradigm. The feature they all have in common is that they introduce some sort of rigidity at the micro level. One model assumes that only a given number of firms can change their price in a particular period. This can be due to several reasons. The firm may be locked into contracts which prevent them from changing their prices. There are also costs which are incurred when changing prices, so-called menu costs. All of these reasons may contribute to the price stickiness. This means that, in any given time period, some firms will have a price which is not optimal for the conditions. Governments can take advantage of this by manipulating the level of demand to induce output responses by firms which are unable to change their prices. Other models attempt to incorporate the fact that markets are not perfectly competitive. Firms in these models are given a degree of market power which allows them to charge a price above the marginal cost. These sorts of rigidities give a theoretical justification for government intervention. It is these sorts of models which are used by the UK Treasury and the Bank of England when deciding on the appropriate monetary policy and when forecasting.