What do you mean "Null"? Turning Null Responses into Quality Responses. - Mimi Kao; Nick Cercone; Wo-Shun Luk (ICDE, 1987)

Providing Quality Responses with Natural Language Interfaces: The Null Value Problem. - Mimi Kao; Nick Cercone; Wo-Shun Luk (IEEE Trans. Software Eng., 1988)