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There is often a need to reduce sample size in clinical trials due to practical limitations and ethical considerations. Better comparability between treatment groups by use of stratification in the design, and use of continuous outcome variables in the evaluation of treatment results, are two methods that can be used in order to achieve this. In this paper the choice of stratification factors in trials of clinical mastitis in dairy cows is investigated, and two score-scales for evaluation of clinical mastitis are introduced. The outcome in 57 dairy cows suffering from clinical mastitis and included in a clinical trial comparing homeopathic treatment, placebo and a standard antibiotic treatment is investigated. The strata of various stratification factors are compared across treatments to determine which other factors influence outcome. The two score scales, measuring acute and chronic mastitis symptoms, respectively, are evaluated on their ability to differentiate between patients classified from clinical criteria as responders or non-responders to treatment. Differences were found between the strata of the factors severity of mastitis, lactation number, previous mastitis this lactation and bacteriological findings. These factors influence outcome of treatment and appear relevant as stratification factors in mastitis trials. Both score scales differentiated between responders and non-responders to treatment and were found useful for evaluation of mastitis and mastitis treatment. [ABSTRACT FROM AUTHOR]