Dohoo I, Martin S, Stryhn H (2003) Veterinary Epidemiologic Research. AVC Inc, Charlottetown, Prince Edward Island, Canada. pp 27 - 52.
Cameron A (1999) Survey Toolbox A Practical Manual and Software Package for Active Surveillance of Livestock Diseases in Developing Countries. Australian Centre for International Agricultural Research Monograph No. 54. pp 37 - 47.
Cannon R and Roe R (1982) Livestock Disease Surveys: A field Manual for Veterinarians, Australian Bureau of Animal Health, Canberra, Australia. pp 2 - 9 (surveys and sampling methods).
Baldock F (1998) What constitutes freedom from disease in livestock? Australian Veterinary Journal 76: 544 - 545.
Stevenson MA (2008) Introduction to Veterinary Epidemiology: 227.407 Study Guide. Massey University, Palmerston North, New Zealand. pp 60 - 67.
Presentations
Probability and non-probability sampling methods.
Creating sampling frames and sampling techniques.
Sources of error when sampling and strategies to reduce error.
I'm designing a study - how many samples should I take?
Estimating population parameters on the basis of a sample.
Review and comment on the sampling protocol to declare Belize free from classical swine fever.
Exercises
Carefully read the following examples and then choose your preferred sampling unit. Explain your choice.
You wish to estimate the economic losses arising from lameness in sheep in a region. You decide to conduct a survey to estimate the prevalence of lameness in sheep in the region.
You wish to conduct a survey to determine the incidence of foot-and-mouth disease (FMD) outbreaks in pigs from an intensively farmed endemic region of a country during the last year.
You believe that poor stockyards and overcrowding of cattle in abattoirs before slaughter is contributing to carcass bruising. You plan to conduct a study to investigate this proposed risk factor.
It is frequent in animal production systems to divide animals into separate groups. Dairy farms (for example) manage lactating and non-lactating stock as separate groups. These divisions can make the collection of a representative sample from a population difficult. Give three examples from livestock enterprises where free mixing of animals is prevented. List all of the sub-groups that may be present in each. How would you obtain a representative sample from each enterprise?
Suppose you wish to determine the prevalence of disease within the pig population of a region. Previous surveys have indicated that 70% of the region's pigs are located in very large, intensive specialised pig farms, 20% of pigs are found within smaller farming units (frequently as a secondary industry on large dairy farms), and 10% of pigs are kept singly within small plots around towns (by people whose major occupation is not farming). With proportional stratification, a sample would be selected at random from within each stratum such that the aggregated sample would consist of 70% pigs obtained from the large intensive farms, 20% pigs obtained from the smaller pig farms, and 10% pigs obtained from small plots near towns. Explain why it is important for each stratum of pigs to be represented in this sample for the prevalence survey.
Assume that the disease that you are investigating is leptospirosis: combine your knowledge of leptospirosis with the description of the farming systems. Is the epidemiology of leptospirosis likely to vary between the different strata?
It is decided to do a survey to estimate the prevalence of disease X in a population of cattle. Three experts are asked for their opinions about the expected prevalence and they reply: 75%, 50% and 25%. Assuming that there are 1 million head of cattle in the study area, a desired precision of 5% and a desired confidence level of 95%. Calculate the needed sample size according to the three expert opinions.
When prevalence is unknown and you have absolutely no idea about its expected value, what prevalence estimate should you use for the sample size calculation?
Serological surveillance for a disease of poultry is to be conducted in a population of 15,000 villages. Each village contains between 10 and 2100 eligible birds. The mean number of birds per village is 750. The requirement is to be 95% certain of declaring a village positive for disease if the within-village prevalence is greater than or equal to 5% and the between-village prevalence is greater than or equal to 1%. If all birds were tested in sampled villages, how many villages would need to be sampled to achieve the required probability of detection?
Example examination questions
Using examples, write brief notes on sampling methods used to select participants in epidemiological studies (2001 written).
Briefly describe the essential features and application of stratified random sampling (2002 written).
Additional reading
Bennett S et al. (1991) A simplified general method for cluster-sample surveys of health in developing countries. World Health Statistics Quarterly 44: 98 - 106.
Cameron A (1999) Survey Toolbox A Practical Manual and Software Package for Active Surveillance of Livestock Diseases in Developing Countries. Australian Centre for International Agricultural Research Monograph No. 54. pp 48 - 81.
Noordhuizen J et al. (1997) Application of Quantitative Methods in Veterinary Epidemiology. Wageningen Pers, Wageningen. pp 31 - 62.
Kelsey J et al. (1986) Methods in Observational Epidemiology. Oxford University Press, New York. pp 254 - 284.
Humphry R et al. (2004) A practical approach to calculate sample size for herd prevalence surveys. Preventive Veterinary Medicine 65: 173 - 188.
Ziller M et al. (2002) Analysis of sampling strategies to substantiate freedom from disease in large areas. Preventive Veterinary Medicine 52: 333 - 343.
Otte J, Gumm I (1997) Intra-cluster correlation coefficients of 20 infections calculated from the results of cluster-sample surveys. Preventive Veterinary Medicine 31: 147 - 150.
