Nonresponse bias in change estimation: a national forest inventory example

Nonresponse in national forest inventories primarily occurs in forested areas due to accessibility issues or where hazardous conditions exist. As with all surveys, nonresponse has the potential to impart empirical bias into sample-based estimates and care should be taken to minimize any effects. A less-studied aspect is the effects of nonresponse when estimating change between two points in time. In this study, potential bias in change estimates was evaluated using imputed values for nonresponse inventory plots to compare differences between response and nonresponse means. Analysis of forest area and tree biomass density attributes revealed that systematic differences in probabilities of nonresponse that occur due to ownership type and forest/nonforest status produce overall estimates of change that are too small. The empirical bias appears to worsen as nonresponse rates increase. Underestimation of change inhibits detection of statistically significant shifts in forest resource attributes and concurrently thwarts effective management and policy responses. Thus, further study to ameliorate this issue is warranted, including improved strategies for defining populations and strata to better conform to nonresponse assumptions and/or alternative estimation methods that account for differential nonresponse probabilities due to ownership or forest/nonforest status.