Pesticide Usage in Scotland: Protected Edible Crops 2015 - Summary Report

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 indication of the precision of estimates the report includes standard errors. 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 17) 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 pesticides among holdings. This is common in the protected edible report as, unlike other surveys, holdings grow a large variety of different crops which receive very different pesticide treatments. For this reason, the standard errors for protected edible crops are much higher than for the other pesticide usage reports (89 and 86 per cent for area and weight respectively) and as such data users should exercise caution when drawing conclusions from the data.

Table 17 Relative standard errors

Estimated standard errors for the area treated (ha) with pesticide and for weight of active ingredient (kg) applied