Pesticide usage in Scotland: Outdoor Vegetable Crops 2021

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, relative standard errors (RSE) for total pesticide use on vegetable crops for both area and weight have been calculated. 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 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 vegetable crops in 2021 was nine per cent for both area and weight, compared with five per cent for both area and weight in 2019. In contrast to previous years, RSE are not reported for constituent crop groups due to the small sample size of some crops encountered in some regions which presented issues with RSE calculation. Alternative methods for calculating RSEs for minor crops with low sample numbers are being investigated.