The dual-currency system entrenches division and inequality, and separates a privileged few from everyone else. Perhaps worst of all, Cuban citizens touched by international tourism cannot help but notice that foreigners have hard currencies and spend convertible pesos freely. By contrast, hardly any Cubans can acquire hard currency (unless they are senior in the party or work in hotels, or have income from casas particulares or paladares), and they certainly cannot spend it outside Cuba. Financial apartheid prevails on the island between Communist Party members and the apolitical majority, and internationally between the Cuban people and the rest of humanity.

The existence of the two currencies also complicates any appraisal of Cuba's economic record, particularly of its standing in international league tables of growth or incomes per head. In the Sixties such left-wing worthies as the prominent Cambridge economist Joan Robinson, wrote with enthusiasm about Cuba's socialist transformation. No one can any longer be that silly. But there is ample scope for debate about exactly how bad its performance has been. The consensus is that the last 20 years have seen modest but meaningful progress, and that incomes and consumption per head are higher than they were 50 years ago. The counter-argument to be made here is that Cuba must be poorer now than in the late Fifties.

Cuba's isolationism and idiosyncrasy bedevil analysis. Critically, what is the correct exchange rate to use when converting the value of Cuban output in national pesos into something that has international meaning? By official decree, the national Cuban peso (CUP) can be exchanged into the convertible peso (CUC) at a fixed rate of 25 to one, while one CUC is supposed to have the same value as one US dollar. According to the Havana Times of August 6, 2013, the average monthly income is 466 CUP. On this basis Cubans typically earn less than $25 a month and under $300 a year. Now Cubans are poor by the standards of other countries, with foreign travel unthinkable for the great majority of the population. But the degree of poverty is overstated by the $25-a-month figure. Sure, Cuba is a grubby, run-down, tired sort of place. All the same, it is plain to any observer that living standards are appreciably higher than implied by expressing local values at the CUC exchange rate. Because a high proportion of consumption is financed by coupon entitlements to subsidised goods and services, and so lies outside the market sector altogether, methods of comparison that use current prices and exchange rates are misleading.

The difficulty is to judge how much uplift should be given to the $25-a-month figure to arrive at a number that makes sense in international statistics. The Central Intelligence Agency ought to be an authority, given the prominence of Cuba in American foreign policy and the trade embargo that the US has maintained for more than five decades. In its World Factbook the CIA somehow reaches an annual income number of over $10,000 a head in terms of so-called "purchasing power parity", which implies that living standards are similar to Brazil's. (Lack of space prohibits a detailed technical discussion of the difference between GDP "at current prices and current exchange rates", and GDP "at purchasing power parity", or PPP. In short, economists agree that international comparisons of GDP need to make allowance for non-traded services, which have a large weight in the typical consumption basket. The relative prices of such services and traded goods vary enormously between countries, and so also do estimates of GDP in PPP terms relative to those in current prices and exchange rates. Both approaches have their uses.)