parity (PPP) tests are complicated by nominal exchange rate noise. (This noise would be an econometric problem, even assuming that the exchange rate volatility is a pure error term). There may be some real economy border effects between countries which limit the flow of tradables or people. Monetary effects, and exchange rate movements can affect the real economy and complicate the picture, a problem eliminated if comparing regions that use the same currency unit. Taxes are very different in many countries, whereas in a same country taxes are usually equal or similar. A pint of pub beer is famously more expensive in the south of England than the north, but supermarket beer prices are very similar. This may be treated as anecdotal evidence in favour of the Balassa–Samuelson hypothesis, since supermarket beer is an easily transportable, traded good. (Although pub beer is transportable, the pub itself is not.) The BS-hypothesis explanation for the price differentials is that the 'productivity' of pub employees (in pints served per hour) is more uniform than the 'productivity' (in foreign currency earned per year) of people working in the dominant tradable sector in each region of the country (financial services in the south of England, manufacturing in the north). Although the employees of southern pubs are not significantly more productive than their counterparts in the north, southern pubs must pay wages comparable to those offered by other southern firms in order to keep their staff. This results in southern pubs incurring a higher labour cost per pint served. Empirical evidence on the Balassa–Samuelson effect Evidence for the Penn effect is well established in today's world (and is readily observable when traveling internationally). However, the Balassa–Samuelson (BS) hypothesis implies that countries with rapidly expanding economies should tend to have more rapidly appreciating exchange rates (for instance the Four Asian Tigers); conventional econometric tests yield mixed findings for this prediction. In total, since it was (re)discovered in 1964, according to Tica and Druzic (2006) the HBS theory "has been tested 60 times in 98 countries in time series or panel analyses and in 142 countries in cross-country analyses. In these analyzed estimates, country specific HBS coefficients have been estimated 166 times in total, and at least once for 65 different countries". Many papers have been published since then. Bahmani-Oskooee and Abm (2005) & Egert, Halpern and McDonald (2006) also provide quite interesting surveys of empirical evidence on