The Family Expenditure Survey (FES) in Northern Ireland (Archive study number 33240) is the responsibility of Policy Planning and Research Unit (See Jardine). In 1967 the Northern Ireland sample was collected separately from the UK for the first time. The sample was increased to 912 from about 130 to facilitate separate analysis and the exercise was supervised by the then Ministry of Finance. Thereafter a random sample of the Northern Ireland sample was forwarded to the UK FES.

The 1967 NI sample design was a two stage one with stratification by area and population density. Three broad geographic divisions were employed; Belfast, the western counties and the remainder. Outside Belfast urban and rural areas were identified by the local authority classifications and the required number were selected randomly with a probability proportional to their size. From each area selected in the first stage, together with the Belfast County Borough. addresses were selected from the list in the Valuation Office using a random start and constant interval for each area, with the number of addresses varying between strata - the 1967 Report gives further details.

Although local government was reorganised in 1973, the above sample design continued to operate until 1975. Thereafter the electoral wards of the 26 District Councils became the primary sampling units (psu) of the new design, which again was two stage and stratified by population density and area (the geographic stratification was similar to the earlier design).

Wards within each of the three geographic strata were subdivided into three equal groups based upon a population density criterion, giving nine strata in all. The first stage of the sampling consisted of the selection of wards with a probability proportional to size. From each of the Belfast strata, two wards were chosen per quarter, against six for the remaining strata. The second stage of sampling again used the Valuation List, with the number of addresses varying between strata - details are given in the 1976 Report. The current NI design, adopted in the mid 1980's, maintains the geographic stratification but now consists of a simple random sample within each strata with 20% of addresses drawn from the Belfast stratum, 45% from the East with the remainder from the West.

In Great Britain, the FES (Archive study number 33057) sample design was and is significantly different, reflecting the considerably greater population. Up to 1985 a four stage, stratified rotating design with a uniform overall sampling fraction was employed. The local authority areas of GB were stratified into (i) 16 standard regions, (ii) type, based on population density and (iii) economic character, based upon rateable value.

Each PSU selected was used for 4 quarters with one quarter of the total PSU's being reselected each quarter. (More details are given in Kemsley et al. 1980).

Along with other household surveys conducted by the Office of Population Censuses and Surveys. the FES in GB changed its sample design in 1986 and began using the small user postal address file with the postcode postal sector as its psu instead of the electoral ward (see Wilson and Elliot, 1987). Some 672 postal sectors were randomly selected during the year after being arranged in strata comprising standard region, area type and the proportion of owner-occupiers and proportion renters according to the 1981 Census. The result was a less clustered sample design which it was hoped would increase the precision of estimates.

Clearly the contemporary GB and NI FES employ a radically different sample design. The extent to which this affects the relative accuracy of the two surveys would be a research project in itself. However, a more pressing issue is provided by the response rates to the two surveys, which are displayed in Fig 1. The UK response rate has fluctuated about 69% whereas in N. Ireland the response rate steadily declined from 80% in 1967 to 53% in 1989. This could be related to the political situation - indeed in the years 1970-74 a total of 183 addresses were abandoned due to civil unrest.

A variable response rate naturally exacerbates the problems associated with the differential response rates by social and economic category. UK response rate rose from 67% in 1980 to 72% in 1981. As a result the proportion of households headed by a person of 30 years and under 50 rose by 1 percentage point (Employment Gazette, Sept, 1982). If the 1980 Survey were reweighted by the 1981 figures the result would be a 0.8% increase in the total expenditure, though only a 0.1% increase in total expenditure per person. (Employment Gazette, Dec, 1982).

With the smaller sample size in N. Ireland, effects can be more dramatic. In 1980 (as in 1978) there were about 1.5% more pensioners than might be expected, thus depressing the estimates of average household size, income and expenditure - the latter two were possibly 4% higher than the 1980 FES had suggested (NI FES Report for 1980).

Turning to the questionnaire itself, the NI FES uses the same forms as GB except, from April 1989, for those parts covering rates and the community charge. A significant development in NI was the addition from the 1988 survey onwards, of a voluntary question concerning the religious denomination of the respondent. In Table I the residual responses include other religious denominations as well as those who refused to answer the question, so it would appear that whatever the reticence to cooperate with the FES overall, this question elicits a strong response.

The FES data for Northern Ireland are deposited in the Archive. There are special access conditions which require that details of the proposal research be submitted. On receipt of approval the data is forwarded to the researcher in either SIR format or, if required, in SPSS format. The FES data are structured with households at the top of the hierarchy, individuals at the base and either tax units or benefit units in between.

References

CSO Annual Abstract of Statistics, 1989.
CSO, Family Expenditure Survey, Annual Reports.
Dept. of Employment, "Pattern of household spending in 1981", Employment Gazette, Vol.90, Dec.1982, pp.521-526.
Dept. of Employment, "Household spending in 1981", Employment Gazette, Vol.90, Sept.1982, pp.394-5.
Kemsley, W.F.F., Redpath, R.U., and Holmes, M. (1980), Family Expenditure Survey Handbook, London; HMSO, 1980.
NI Dept of Finance and Personnel, NI Annual Abstract of Statistics, 1986.
NI Department of Finance and Personnel, FES reports: 1967-81 annual, since then PPRU Monitor No 3/88; No 2/89; No 1/91.
Wilson, P.R. and Elliot, D.J. (1987) "An evaluation of the Postcode Address File as a Sampling Frame and its Use within OPCS", Journal of the Royal Statistical Society, Series A, Vol.150, Part 3, pp.230-240.