Pesticide usage - soft fruit crops 2018: survey results

Appendix 5 - Standard errors

The figures presented in this report are produced from surveying a sample of holdings rather than a census of all the holdings in Scotland. Therefore the figures are estimates of the total pesticide use for Scotland and should not be interpreted as exact. To give an idea of the precision of estimates, the report includes relative standard errors (RSE) (Table 35). Standard errors are produced using the raising factors. An overall variance is calculated by summing the variance estimates for individual strata (region and size group) multiplied by the square of their raising factors. These variance estimates include a finite population correction. The overall standard error is calculated from the overall variance by taking its square root. This method of standard estimation was implemented as it is both relatively straightforward and has advantages over ratio estimator methods when within-strata sample sizes are small.

Standard errors are expressed as percentage relative standard errors (Table 35) for both total pesticide use by area treated and for weight applied. Larger relative standard errors mean that the estimates are less precise. A relative standard error of 0 per cent would be achieved by a census. A relative standard error of 100 per cent indicates that the error in the survey is of the same order as the measurement. Relative standard errors may be reduced with larger sample sizes. However, larger relative standard errors can also result from greater variability in pesticide use among holdings.

The RSE for estimates of total pesticide use on soft fruit crops (protected and non-protected) was 10 per cent for area and 13 per cent for quantity (Table 35)

The RSE for constituent protected and non-protected crop groups varied from 11 to 88 per cent for area and 13 to 52 per cent for weight (Table 36), varying with sample size and uniformity of pesticide regime encountered. However, due to insufficient data, RSE values could not be calculated for all strata and overall RSE values for protected and non-protected soft fruit crops should be treated with caution.