Appendix 4 – Survey methodology
Sampling and data collection
Using the June 2015 Agricultural Census (10), a sample was drawn representing vegetable cultivation in Scotland. The first sample was selected from holdings growing any vegetable crops excluding vining peas, and the second from holdings known to have grown vining peas. Two samples were taken to achieve a better representation of all vegetable crops, as most vining pea crops are grown on farms growing arable crops rather than vegetable crops.
The country was divided into 11 land-use regions ( Figure 39). Each sample was stratified by these land-use regions and according to holding size. The holding size groups were based on the total area of either vegetable or vining peas crops grown. The sampling fractions used within both regions and size groups were based on the areas of relevant crops grown rather than number of holdings, so that smaller holdings would not dominate the sample.
The survey covered pesticide applications to vegetable crops where all or the majority of the growing season was in 2015. As well as recording treatments applied directly to the crop, data was also collected on land preparation treatments prior to sowing or planting the crop.
Following an introductory letter and phone call, data were collected by either personal interview during a visit to the holding or during a phone interview or by email. Where necessary, information was also collected from agronomists and contractors. In total, information was collected from 60 holdings growing vegetable crops and 30 holdings growing only peas ( Tables 37 & 38). These 90 holdings represent 13 per cent of the total crop area grown.
National pesticide use was estimated by ratio raising. This is a standard statistical technique for producing estimates from a sample. It is the same methodology used by the other UK survey teams and has been used for all historical datasets produced by the Pesticide Survey Unit, allowing comparability over time. The sample data were multiplied by raising factors ( Tables 43 & 44). These factors were calculated by comparing the sampled area to the areas recorded in the Agricultural Census within each region and size group. An adjustment ( Table 45) was made for each crop within each region by applying the raising factors to the sample area of each crop grown and comparing this with the census area. This adjustment modifies the estimate to take into account differences in composition of crops encountered in the sample and those present in the population. A second adjustment was necessary for some crops which were present in the population, but were not encountered in the sample in some strata.
Figure 39 Land use regions of Scotland (12)
Changes from previous years
There are a number of changes which should be noted when comparing the 2015 data with the previous survey.
The term active substance is now used instead of active ingredient which appeared in previous reports. These changes make the Scottish reports consistent with the UK pesticide usage reports.
In 2015, all biopesticides have been grouped separately. In previous reports, these were presented under the category of biological control agents within the insecticides and molluscicides table. However, as biopesticides require to be authorised like other pesticides, they can have a range of different functions including fungicides and insecticides and their rates of application can be collected, they are now being treated separately. Biopesticide values have been re-calculated separately from biological control agents for the previous reports to allow for accurate comparisons. No biological control agents were recorded in 2015.
Another change to note is that sulphur was previously reported as a fungicide. It is now reported in a category on its own to acknowledge that as well as being used as a fungicide, it has other functions in some crops. In order to allow comparison with previous surveys, fungicide data from the 2013 and 2011 surveys included in this report have been recalculated to exclude sulphur.
The 2015 report contains a number of new details to help improve data quality for users. Data relating to the average number of applications for each crop and type of pesticide have been included in Table 1 and Figures 11 and 12. Details relating to pesticide application timings for each crop have been included in the pesticide usage section. Fungicides and herbicides have been classified into groups according to their mode of action in Tables 30- 31- 32. Data on Integrated Pest Management activities ( i.e. non-chemical methods to control pests, weeds and diseases) has been collected from the growers and is reported in Appendix 6.
Data quality assurance
The dataset undergoes several validation processes as follows; (i) checking for any obvious errors upon data receipt (ii) checking and identifying inconsistencies with use and pesticide approval conditions once entered into the database (iii) 100 per cent checking of data held in the database against the raw data. Where inconsistencies are found these are checked against the records and with the grower if necessary. Additional quality assurance is provided by sending reports for review to members of the Working Party on Pesticide Usage Surveys and other agricultural experts. In addition, the Scottish pesticide survey unit is accredited to ISO 9001:2008. All survey related processes are documented in Standard Operating Procedures ( SOPs) and our output is audited against these SOPs by internal auditors annually and by external auditors every three years.
Main sources of bias
The use of a random stratified sample is an appropriate survey methodology. A stratified random sample, grouped by farm size and region, is used to select holdings used in this survey. Sampling within size groups is based on area rather than numbers of holdings, so that smaller size groups are not over-represented in the sample. The pesticide survey may be subject to measurement bias as it is reliant on farmers/growers recording data accurately. As this survey is not compulsory it may also subject to non-response bias, as growers on certain farm/holding types may be more likely to respond to the survey than others. Reserve lists of holdings are held for each stratum to allow non-responding holdings to be replaced with similar holdings.
Experience indicates that stratified random sampling, including reserves, coupled with personal interview technique, delivers the highest quality data and minimises non-response bias.