[Excerpt] In our recent paper (Shilling et al. ), we examined the rent adjustment process for 17 U.S. office markets and provided the first estimates of the natural vacancy rates. In his comment, Voith  argues that our specification of the rent-vacancy relationship is biased since we introduced an interaction term to capture the effect of risks associated with higher vacancy levels. He suggests that an alternative specification would be to enter a vacancy squared term to capture this risk effect.
Shilling, J. D., Sirmans, C. F., & Corgel, J. B. (1992). Natural office vacancy rates: Some additional estimates[Electronic version]. Retrieved [insert date], from Cornell University, School of Hotel Administration site: