The error in objective analysis methods that are based on corrections to a first guess field is considered. An expression that gives a decomposition of the error into three independent components is derived. To test the magnitudes of the contribution of each component a series of computer simulations was conducted. grid-to-observation point interpolation schemes considered ranged from simple piecewise linear functions to highly accurate spline functions. The observation-to-grid interpolation methods considered included most of those in present meteorological use, such as optimum interpolation and successive corrections, as well as proposed schemes such as thin plate splines, and several variations of these schemes. The results include an analysis of cost versus skill; this information is summarized in plots for most combinations. The degradation in performance due to inexact parameter specification in statistical observation-to-grid interpolation schemes is addressed. The efficacy of the mean squared error estimates in this situation is also explored. (Author)