A subtle difference between propositional and relational data is that in many
relational models, marginal probabilities depend on the population or domain
size. This paper connects the dependence on population size to the classic
notion of projectivity from statistical theory: Projectivity implies that
relational predictions are robust with respect to changes in domain size. We
discuss projectivity for a number of common SRL systems, and identify syntactic
fragments that are guaranteed to yield projective models. The syntactic
conditions are restrictive, which suggests that projectivity is difficult to
achieve in SRL, and care must be taken when working with different domain
sizes.

本文研究了命题数据和关系数据之间微妙差别，指出在许多关系模型中，边缘概率取决于人口或领域的大小，将此与统计理论中的经典概率密度函数基本概念建立联系：Projectivity 意味着关系预测对领域大小的变化具有鲁棒性，讨论了许多常见 SRL 系统的 projectivity，找出了保证产生 projective 模型的语法片段，但语法条件是限制性的，这表明在 SRL 中难以实现 projectivity，应注意区分不同领域大小。