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Estimates of Mean Differences Necessary for Significance between Pigs in Group Feeding Experiments

United States Department of Agriculture

Abstract

A question frequently raised in the course of group-feeding experiments with pigs is how large a mean difference is needed between groups to preclude the probability that it was accidental. To answer this question precisely, it is usually necessary to know not only how many individuals there were in each group, but also how much variation there was in the two samples. Once this information has been obtained, the mean difference is calculated as follows:

Where (₁–₂) is the mean difference, n₁ and n₂ are the number of individuals in the two groups, s is the pooled Standard deviation of the two groups and t is the ratio of a deviation to its estimated Standard error.

Since n₁ and n₂ are usually known and tables giving the probability of a deviation falling outside the range ± t can be found in many statistical text-books, it follows that s is the only statistic that has to be calculated in order to solve for (₁–₂).