Pesticide Usage in Scotland: Rodenticides on Grassland & Fodder Farms 2017

Appendix 4 - Survey methodology

Sampling and data collection

Using the June 2017 Agricultural Census ^{[ 7]} two samples were selected, one taken from holdings with grassland, the second from holdings growing fodder crops. For the purpose of sampling, the country was divided into 11 land-use regions ^{[ 8]} (Figure 11) and five size groups. The size groups were different for grassland and fodder crops (Table 7 & 9 respectively) and were based on the total areas of crops grown on the holding. Holdings were chosen at random within each of these strata, with the number of holdings selected being proportional to the total area of crops grown. Sample size groups were based on crop area rather than number of holdings, so that smaller holdings did not dominate. This stratification was designed to take into account differences in rodenticide use in relation to geography and farm size when making estimates of national use.

The survey covered rodenticide use during the 12 month period January to December 2017. Following an introductory letter, data was gathered from farms growing fodder crops by telephone interview or email. For the grassland survey a postal questionnaire was sent out which farmers completed and returned, followed up with a phone call where necessary. When rodenticides were applied by a pest control professional ( PCP) and data were not available from the farmer the information was obtained directly from the contractor. If it was recorded that rodenticides were used but product data were not obtainable from either the farmer or PCP this was recorded as unspecified rodenticide use.

In total, information was collected from 635 holdings. These holdings represent 6.3 per cent of the 2017 Scottish fodder crop holdings (Table 8) and 1.2 per cent of the grassland holdings (Table 6). The data collected were; rodenticide user, product(s) used, bait type, weight applied, target and season of use. Information about use of non-chemical rodent control methods was also recorded.

All farmers and PCPs encountered in the survey were also asked to respond to a simple questionnaire containing questions relating to whether they had received training in use of rodenticides and their self-reported compliance with best use practice for rodenticides. Farmers were also asked to answer questions about their knowledge of rodenticide stewardship and details about their farm, such as whether they kept livestock or had a grain store.

It should be noted that, in relation to all data collected, responses are as reported by the rodenticide users and no attempt has been made to check their accuracy

Figure 11 Land use regions of Scotland ^{[ 8]}

Estimation of national rodenticide use

The figures presented in this report are produced by surveying a sample of holdings rather than conducting a census of all the holdings in Scotland. Therefore the figures are estimates of total rodenticide use for Scotland and should not be interpreted as exact.

National rodenticide use (holdings using rodenticides, rodenticide occurrence and weight) was estimated from the sample data by ratio raising. This is a standard statistical technique for producing estimates from a sample. This method involves multiplying the sample data by a factor dependent on the number of farms within each region and size group to match the data recorded in the relevant June Agricultural Census for arable crops. Due to small sample sizes the data from some regions were merged and a secondary adjustment factor was applied to the raising factors to account for region and size groups for which no holdings were sampled. Details of regions, size groups, raising and adjustment factors are presented in Tables 10 and 11.

The remainder of the data (use of non-chemical control methods, details of farm operation, compliance with best practice and knowledge of rodenticide stewardship) are unraised and represent the information collected from the sample.

Changes from previous years

In previous reports in this series, data about the type of rodenticide user ( i.e. farmer or PCP) were based on the proportions encountered in the sample surveyed. Whilst this was made clear in the reports, it was in contrast to the weight and occurrence data presented, which were estimates of use in the total population of arable farms. Due to database improvements, population estimates of user type are now available and replace the sample data in this report for both 2017 and historical data. This has resulted in differences in 2013 user data presented in this report and in the original report.

For the first time in this series of surveys we asked questions about target of rodenticide use, non-chemical controls employed, knowledge of rodenticide stewardship, uptake of stewardship affiliated training and how rodenticides had been purchased post-stewardship.

Statistical analyses

As estimates are based on a random stratified sample of farms in each survey year and individual farms may be sampled more than once in the time series, there is no simple method of statistical comparison for estimated rodenticide use on arable farms over time. However, the percentage of farms using rodenticides, the percentage of farms on which baiting was conducted by PCPs and the percentage occurrence of first and second generation compounds have been analysed using Pearson's chi-squared test. The percentage occurrence of first and second generation compounds was analysed using the number of holdings as a base. These conservative analyses do not take into account the stratification, finite population sampling or common farms between years and are therefore less likely to find significant differences. All significant differences are highlighted in the text and tables of this report.

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 farmer if necessary. Additional quality assurance is provided by sending reports for independent review. In addition, the Scottish pesticide survey unit is accredited to ISO 9001:2015. All survey related processes are documented in Standard Operating Procedures ( SOPs) and output is audited against these SOPs by internal auditors annually and by external auditors every three years.

Main sources of bias

These surveys may be subject to measurement bias as they are reliant on respondents recording data accurately. As surveys are not compulsory they may also be subject to non-response bias, as some farmers and PCPs may be more likely to respond than others. However, the use of a random stratified sample is an appropriate survey methodology and reserve lists of farms are held for each stratum to allow non-responding farms 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.