Taxes are collected with lags in almost all countries. In industrial nations, average collection lags—which measure the time between the moment taxes due are calculated and the moment they are actually paid to the fiscal authority—vary from one month in some cases and for particular sources of taxation (such as income taxes that are withheld at the source) to six to ten months in other cases (such as indirect taxes). In developing countries, by contrast, average collection lags may be substantially higher. The share of revenue generated by taxes collected with progressive rates and withheld at the source is small, and taxes (such as import duties and excises) are often levied at specific rates. In such conditions an increase in the inflation rate will bring a fall in real conventional tax revenue, the extent of which will depend on the average collection lag and the prevalent tax burden, that is, the initial ratio of taxes to aggregate output. Formally, let n denote the average lag in collection of conventional taxes measured in months, and let πM denote the monthly inflation rate. The real value of conventional tax revenue at an annual inflation rate of πs is given by