The Estimators Used in the New Mexico Inventory: Practical Implications of Nonresponse Being 'Truly' Random Within Each Stratum

• Activity Number: 293
• Type: Topic Contributed
• Date/Time: Tuesday, August 6, 2013 : 8:30 AM to 10:20 AM
• Sponsor: Section on Statistics and the Environment
• Author(s): Paul Patterson*+ and Sara Goeking
• Companies: US Forest Service and US Forest Service
• Keywords: forest resource monitoring ; nonresponse ; stratification ; variance estimation

Abstract

The Forest Inventory and Analysis Program (FIA) of the US Forest Service is designed to provide state and national assessments of forest conditions. In New Mexico the sample included a large proportion of nonresponse, most of which occurs on private lands where access was denied. FIA's estimation process uses post stratification and assumes that nonresponse occurs at random within each stratum. However, the probability of nonresponse can vary within each stratum because some plots are not field-sampled if they are identified as non-forest using remote sensing techniques. Plots not sent to the field cannot result in nonresponse and therefore consideration must be given to forming a separate stratum from plots sent to the field. In this situation the standard FIA variance estimators are not applicable. We derive the variances of the estimators used in the New Mexico inventory, propose estimated variances and explore the statistical properties of the estimated variances. We assess the size of the bias introduced by not stratifying on the remotely sensed non-forest plots versus field-sampled plots, and we propose guidelines for when such stratification is needed.