Since the probabilities of selection are constant within an SSU for 1997, these adjustments were applied at the SSU level. In some cases, a single PSU comprising all or part of a large metropolitan area was large enough in population to be a stratum by itself. The first-stage ratio adjustment factor was 1.0 for all observations in SR PSUs. In other strata, one PSU was selected from among two or more PSUs in the stratum. Fuels are under-represented if the percentage of households using the fuel is lower in the selected NSR PSUs than the percentage in the set of all PSUs in the NSR strata.
Fuels are over-represented if the opposite occurs.
The second-stage ratio adjustments are used to improve the accuracy of the estimates of the number of households using data obtained from the Bureau of the Census as control totals. The RECS can be used to produce an estimate of the number of households in the country, but the Bureau of the Census produces much more accurate estimates. The first priority is the accuracy of estimates for the number of households for the nine Census divisions and for the four largest States. The ratio adjustment process was carried out in three steps. In step one, the population was divided into 15 geographical cells.