Data analysis 2

Readings

Hypothesis testing, p-values and confidence intervals

• Curtis CR, Salman MD, Shott S (1990) Statistics simplified - P values. Journal of the American Veterinary Medical Association 197: 318 - 320.
• Shott S (1990) Statistics simplified - confidence intervals. Journal of the American Veterinary Medical Association 197: 576 - 578.
• Thrusfield M (2005) Veterinary Epidemiology. Blackwell Science, London. pp 247 - 265 (estimation and hypothesis testing).
• Dohoo I, Martin S, Stryhn H (2003) Veterinary Epidemiologic Research. AVC Inc, Charlottetown, Prince Edward Island, Canada. pp 131 - 136 (hypothesis testing and confidence intervals).
• Petrie A, Watson P (2005) Statistics for Veterinary and Animal Science. Blackwell Science, London. pp 69 - 75 (hypothesis testing).

Statistical tests

• Curtis CR, Salman MD, Shott S (1991) Statistics simplified - comparing means. Journal of the American Veterinary Medical Association 198: 62 - 65.
• Shott S (1990) Statistics simplified - comparing proportions. Journal of the American Veterinary Medical Association, 197: 1460 - 1462.
• Shott S (1990) Statistics simplified - association. Journal of the American Veterinary Medical Association 198: 404 - 407.
• Shott S (1990) Statistics simplified - nonparametric statistics. Journal of the American Veterinary Medical Association 198: 1126 - 1128.
• Petrie A, Watson P (2005) Statistics for Veterinary and Animal Science. Blackwell Science, London. pp 78 - 88 (t-test and hypothesis testing), pp 90 - 92 (F-test for variance), pp 101 - 112 (Chi-squared test and hypothesis testing) pp 138 - 150 (non-parametric tests).

Standardisation

• Thrusfield M (2005) Veterinary Epidemiology. Blackwell Science, London. pp 63 - 65.

Stratified analysis, clustering, and sources of error

• Curtis CR, Salman MD (1990) Statistics simplified - power and sample size. Journal of the American Veterinary Medical Association 197: 838 - 840.
• Thrusfield M (2005) Veterinary Epidemiology. Blackwell Science, London. pp 278 - 280 (Mantel-Haenzel procedure).
• Dohoo I, Martin S, Stryhn H (2003) Veterinary Epidemiologic Research. AVC Inc, Charlottetown, Prince Edward Island, Canada. pp 29 - 30, 40 (types of error), pp 459 - 471 (clustering and stratified analysis).
• Curtis CR, Salman MD (1990) Statistics simplified - power and sample size. Journal of the American Veterinary Medical Association 197: 838 - 840.
• Petrie A, Watson P (2005) Statistics for Veterinary and Animal Science. Blackwell Science, London. pp 175 - 178.

Presentations

• Hypothesis testing, power, type 1 and 2 errors.
• Basic statistical tests: unit of analysis, independence, parametric and non parametric tests (including underlying assumptions, applications and limitations), tests for different data types.
• Stratified analysis and the Mantel-Haenzel procedure.
• 'Tricky data' - paired and matched data, repeated measures on the same animals, clustered data, and autocorrelation.

Exercises

Read the following papers and discuss how clustering was accounted for during the design and analysis stages of the studies:

• Gitau GK, Perry BD, Katende JM, McDermott JJ, Morzaria SP, Young AS (1997) The prevalence of serum antibodies to tick-borne infections in cattle in small holder dairy farms in Murang'a District, Kenya; a cross-sectional study. Preventive Veterinary Medicine 30: 95 - 107.
• Haine D, Boelaert F, Pfeiffer D, Saegerman C, Lonneux J-F, Losson B, Mintiens K (2004) Herd level seroprevalence and risk mapping of bovine hypodermosis in Belgian cattle herds. Preventive Veterinary Medicine 65: 93 - 104.

Example examination questions

1. Define the null and alternative hypothesis. (2005 oral).

2. P values and how they are determined (2005 oral).

3. What are the application, advantageous and limitations of parametric and non-parametric test? Give two examples of each type of test (2005 oral).

4. Briefly describe the essential features, application and limitations of stratified analysis (2005 written).

5. Using examples write brief notes on: (a) p values and significance testing (2003 written); (b) confidence intervals (2002 written); (c) power in epidemiological studies (2001 written).

Additional reading

• Epidemiological Skills in Animal Health, PGFVS Proceedings 143; pp 222 - 236 (analysing epidemiologic data).
• McDermott JJ, Schukken YH, Shoukri MM (1994) Study design and analytic methods for data collected from clusters of animals. Preventive Veterinary Medicine 18: 175 - 191.
• McDermott JJ, Schukken YH (1994) A review of methods used to adjust for cluster effects in explanatory epidemiological studies of animal populations. Preventive Veterinary Medicine 18: 155 - 173.