The Shopper’s Guide to Pesticides ranks pesticide contamination for 47 popular fruits and vegetables based on an analysis of 87,000 tests for pesticides on these foods, conducted from 2000 to 2007 by the U.S. Department of Agriculture and the Food and Drug Administration. Nearly all the studies used to create the list test produce after it has been rinsed or peeled. Contamination was measured in six different ways and crops were ranked based on a composite score from all categories.
The six measures of contamination we used were:

The philosophy behind the guide is simple: give consumers the information they need to make choices to reduce pesticides in their diets. In this spirit, the Guide does not present a complex assessment of pesticide risks, but instead simply reflects the overall load of pesticides found on commonly eaten fruits and vegetables. This approach best captures the uncertainty of the risks of pesticide exposure and the value judgments involved in the choice to buy food with less pesticides.

Pesticides cause many adverse effects in well designed animal studies, from cancer to nervous system damage to reproductive effects. Rather than assign more weight to cancer than birth defects, we simply assumed that all adverse effects are equal. There is a significant degree of uncertainty about the health effects of pesticide mixtures. This ranking takes this uncertainty into account in the most defensible way possible, by simply ranking fruits and vegetables by their likelihood of being consistently contaminated with the greatest number of pesticides at the highest levels.

The produce listed in the Guide was chosen after an analysis of USDA food consumption data from 1994-1996. The 47 selected were those reported eaten on at least one tenth of one percent of all “eating days” in the survey and with a minimum of 100 pesticide test results from the years 2000 to 2007. An eating day is one day of food consumption reported to USDA by one individual, some of whom were followed for three days.

Here is a ranking of 47 different fruits and vegetables, but grapes are listed twice because both domestic and imported samples were reviewed.

Sources:
http://www.foodnews.org/methodology.php
http://www.mercola.com