Table 2. The number (%) of herds at each level of the sampling strategy.
| Herds (cases and controls) | Number (%) |
| Commercial slaughter-pig holdings in England and Wales | 1400 (100%) [16] |
| Herds sampled by BPHS scheme (data for 2010) | 1036 (74% of 1400) [16] |
| Herds with 50 pigs sampled by BPHS on at least 3 occasions prior to October 2008 | 778 (56% of 1400) |
| Number of eligible cases | 121(16% of 778) |
| Number of eligible controls | 306 (39% of 778) |
| Total number of eligible herds | 427 herds (55% of 778; 31% of 1400) |
| Number of dispatched questionnaires | 242 (121 cases, 121 controls) |
| Number of completed questionnaires | 121 (50% of 242; 16% of 778; 9% of 1400) |
| 51 cases (7% of 778) | |
| 70 controls (9% of 778) | |
| Number of herds included in univariable model | 121 |
| Number of herds included in multivariable model | 121 |
The number (%) of herds at each level of the sampling strategy, including the number of eligible case and control herds, as a proportion of the total number of commercial slaughter-pig herds in England and Wales.