It can more accurately reflect the law of natural occurrence of the disease.Many interference factors are encountered, which requires scientific experimental design.Phase I clinical trials: human pharmacological and safety evaluation trials after laboratory and animal studies show promising results for a new drug (therapy).Phase II clinical trial of 10 to 30 patients: preliminary evaluation of drug efficacy, further evaluation of safety.100-300 person Phase III trials: To further determine effectiveness and monitor side effects, usually at multiple sites.1000 to 3000 people Phase IV clinical trials: monitoring conducted after marketing approval to examine the effects and long-term or rare side effects of drugs in different populations under widely used conditions.Vaccines and drug evaluation clinical trials for animal use are equivalent to phase III clinical trials.That is, field effectiveness evaluation test, field trial.Phase I and II clinical trials of field trials, intermediate trials and regional trials are usually carried out in special animal laboratory rooms or animal testing sites in laboratories, which are laboratory epidemiological studies and regarded as preclinical studies.2. Experimental Design (1) Design principles 1. Relatively stable and high morbidity of the experimental site The herd has little mobility and good veterinary service facilities with characteristics consistent with the overall situation 2.The number of samples required for the evaluation of new veterinary biological products should comply with relevant regulations.The State Council promulgated the “Regulations on the Control of Veterinary Drugs” (effective from November 1, 2004). 3. Grouping and randomization Except for different treatment factors, all other factors should be as similar as possible.Objective measurement methods are consistent, and subjective interference is avoided by blind method. 4. Experimental elements treatment factors: some measures imposed by human, the treatment factors and their level should be consistent throughout the whole process of the test.Subject animals: species, class and strain determination.If the tested animals are diseased animals, the diagnosis must be clear and uniform.Experimental effect: to judge by indicators, select effect indicators with strong intuition, high sensitivity and good accuracy.