Updated: Updated to 2014 to 2015 tax year.
The estimates are based on a sample of individuals. Information on Individual Savings Account (ISA) subscriptions and valuations is reported by providers to HM Revenue & Customs on an annual basis, primarily for compliance purposes. As well as providing information on the type of ISA, this gives details of the account holder’s date of birth, gender and National Insurance Number (NINO). This last item of information enables the data to be linked to personal incomes (both earnings and investment income, as taken primarily from the Survey of Personal Income or SPI). It also enables the analysis to be performed at individual rather than at account level.
Source: HMRC