asked the Chancellor of the Exchequer what would be the increase in revenue at outturn prices which would accrue in the current year if tax relief on owner-occupiers' mortgage interest and interest on other loans for house purchase or improvement, were withdrawn; and how much of this additional revenue would be raised from those who would then be paying tax at the higher rates; what are the equivalent figures for the years 1975–76, 1976–77 and 1977–78; and what are the equivalent figures for these years and the current year expressed at 1977 survey prices.

pursuant to the reply [Official Report, 2nd August 1978], gave the following information:The estimated increase in revenue for 1978–79 would be about £1,080 million. Total higher rate tax would increase by about £320 million.For earlier years the estimated increase in revenue would have been as follows:

	£ million
1975–76	865
1976–77	1,050
1977–78	1,055

Figures at 1977 survey prices are not available.Expressed at constant 1977–78 prices using the general index of retail prices—and assuming a 7 per cent. increase in the index between 1977–78 and 1978–79—the figures are:

US DOLLARS
GROSS DOMESTIC PRODUCT AT FACTOR COST PER HEAD OF TOTAL POPULATION
			1976 Based on exchange rates	Based on purchasing power parities	1977(1)Based on exchange rates
United Kingdom	…	…	3,508	4,262	3,822
United States	…	…	7,240	7,240	n.a.
W. Germany	…	…	6,460	5,351	n.a.
Japan	…	…	4,650	4,770	n.a.
Italy	…	…	2,795	3,528	n.a.
France	…	…	5,760	5,474	n.a.
Canada	…	…	7,482	n.a.	7,430(2)
Austria	…	…	4,592	n.a.	n.a.
Finland	…	…	5,431	n.a.	5,640
n.a.=not available
(1) Not adjusted to standardised definitions
(2) GNP at factor cost

Source: 1976 data-National Accounts of OECD Countries 1976 Volume 11977 data-UK-latest published estimates-Canada and Finland-OECD Quarterly National Accounts BulletinEast Germany and Czechoslovakia do not compile estimates of gross domestic product, and data for Libya for 1976 are not yet available.For comparison purposes, the use of market exchange rates to convert to a common unit of measurement can be very misleading since those rates do not reflect the relative internal purchasing power of currencies. A more satisfactory method is to use purchasing power parities; these have been produced by extrapolating to 1976 the comparisons made by the United Nations of relative