7.1 All percentages cited in this report are based on weighted data. The weights applied to the SSA 2015 data are intended to correct for three potential sources of bias in the sample:
- Differential selection probabilities
- Deliberate over-sampling of rural areas
7.2 Data were weighted to take account of the fact that not all households or individuals have the same probability of selection for the survey. For example, adults living in large households have a lower selection probability than adults who live alone. Weighting was also used to correct the over-sampling of rural addresses. Differences between responding and non-responding households were taken into account using information from the census about the area of the address as well as interviewer observations about participating and non-participating addresses. Finally, the weights were adjusted to ensure that the weighted data matched the age-sex profile of the Scottish population (based on 2014 mid-year estimates from the General Register Office for Scotland).
7.3 Adding weights to a sample can affect the sample efficiency. If the weights are very variable (i.e. they have very high and/or very low values) the weighted estimates will have a larger variance. More variance means standard errors are larger and confidence intervals are wider, so there is less certainty over how close the estimates are to the true population value.
7.4 The affect of the sample design on the precision of survey estimates is indicated by the effective sample size (neff). The effective sample size measures the size of an (unweighted) simple random sample that would have provided the same precision (standard error) as the design being implemented. If the effective sample size is close to the actual sample size then we have an efficient design with a good level of precision. The lower the effective sample size, the lower the level of precision. The efficiency of a sample is given by the ratio of the effective sample size to the actual sample size.
7.5 The final weights in 2015 were slightly more variable than in 2014, which may be due to some of the groups being under- or overrepresented to a greater extent in the sample (e.g. males 25-34). In effect, the effective sample size due to weighting is 34%, smaller than the achieved sample size (as compared with 31% in 2014). Please note the weighting is only one of several design elements influencing the final effective sample size, and the overall effect is different for all estimates. The range of the weights, the effective sample size and sample efficiency for both sets of weights are given in Table 10 below.
Table 10 Comparison of final weights in 2014 and 2015 SSA
|Weights||Variance of weights||Achieved sample size||Effective sample size (weighting effect only)|
Email: Donna Easterlow
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