Before revision:  One difficulty in deriving these quantities is that they must be computed on a model-by-model basis, since it is typically not practical to obtain a general reproduction number expressions applicable to a family of related models, especially if some are of different dimensions.
After revision:  One difficulty in deriving these quantities is that they must be computed on a model-by-model basis, since it is typically not practical to obtain a general reproduction number expressions applicable to a family of related models, especially if these are of different dimensions.