Practical Clinical Epidemiology for the Veterinarian

Aurora Villarroel, DVM, MVPM, PhD, DACVPM, served as Assistant Professor of Clinical Sciences at the College of Veterinary Medicine and Extension Veterinarian in the Department of Animal Sciences at Oregon State University in Corvallis, Oregon. She taught epidemiology for 12 years at the veterinary schools of UC Davis, Colorado State University, and Oregon State University, and is currently a consultant at Athyr Vet, LLC.

Preface xi

Acknowledgments xiii

About the companion website xiv

1 Describing health and disease 1

Case definition 1

What is the problem? 2

Who is affected? 4

Where is the disease concentrated? 5

When does disease occur? 5

Types of measurements 5

Counts 5

Proportions 7

Ratios 8

Rates 9

Specific measurements of disease 10

Prevalence 10

Incidence 13

Morbidity 16

Mortality 16

Disease?]specific mortality 16

Case?]fatality 17

2 Basic epidemiology concepts 19

Outcome 19

Risk factor 21

Unit of analysis 22

Types of variables 26

Appropriate statistical analyses for continuous/parametric variables 27

Appropriate statistical analyses for categorical/nonparametric variables 29

Appropriate statistical analyses for multiple samples taken from the same animal 30

Control groups 33

Sample size and P?]value 34

Error and bias 35

Confounding 41

Interaction 42

3 Evidence?]based medicine for the veterinarian 44

Evaluation of a research paper 49

Data presentation in the results 53

Interpretation of results 57

Statistical significance 58

Biological significance 60

4 Study designs 62

Retrospective studies 63

Case-control studies 64

Surveys 65

Cross?]sectional studies 68

Prospective studies 70

Cohort studies 70

Clinical trials 72

Sampling strategies 73

5 Causation versus association 77

Hill's criteria to determine causation 78

Temporal association 78

Consistency of association 79

Specificity of association 79

Dose-response (biological gradient) 80

Biologic plausibility 81

Analogy 81

Measures of association 82

Odds ratio 84

Relative risk 89

Attributable risk 92

6 Diagnostic tests 94

Test quality 95

Accuracy 95

Precision 95

Discrimination ability 98

Test performance 99

Sensitivity 100

Specificity 102

Positive predictive value 104

Negative predictive value 105

Screening 107

Parallel testing 107

Serial testing 107

Gold standard 108

7 Outbreak investigations 109

Definitions 109

Steps in an outbreak investigation 110

Case definition/diagnosis verification 110

Determine the magnitude of the problem 111

Describe the spatial and temporal patterns of disease 112

Analyze potential risk factors 116

Follow?]up 119

Glossary 120

Formulas 125

Final word 128

References 129

Index 134