Structural Zeros versus Sampling Zeros
Probabilistic sequence models estimated from large corpor a typically require smoothing techniques to reserve some probability mass for unobserved v nts. These techniques fail to distinguish between events unobserved due to sampling li mitat ons,sampling zeros , and those unobserved due to structural reasons such as syntacti c constraints, structural zeros . We investigate the use of statistical tests to determine str uctu al zeros, to avoid assigning them probability mass and thereby improve model accuracy. E xperimental results on a context-free parsing task demonstrate the usefulness of th ese techniques.