The central bank operates a managed float of the pound through foreign-currency purchases and daily limits on the trading band. This is intended to smooth volatility related to oil exports and foreign direct investment flows, but from mid-2010 the central bank was defending an increasingly unrealistic premium on the black-market rate; the difference was at times as much as 30%, as a scarcity of foreign exchange and political uncertainty depreciated the black-market rate. In November 2010 the central bank, probably after depleting its minimal reserves (for which data have not been published since March), finally permitted a devaluation of sorts, by permitting banks to purchase foreign currency at a premium of 18.3%-equivalent to the black-market premium. This unusual step was probably temporary. The authorities prohibit banks from disbursing foreign transfers in foreign currency, require a 100% cash margin on most imports and restrict the foreign exchange that can be taken abroad or sent in personal transfers, but these restrictions are being eased in 2011. The official exchange rate is forecast to depreciate to an average of SP2.83:US$1 in 2011 and SP3.11:US$1 in 2012.