Excerpts from the report Summary: Recent increases in agricultural output have prompted agricultural researchers to investigate the roles of weather and technology in crop yields. This study assesses the relative effect of each on corn yields in the Corn Belt between 1929 and 1962. Weather indexes were constructed for all States of the Corn Belt and the Corn Belt as a whole. These measures were constructed from corn variety test data, and were used to adjust yield and output series for the influence of weather by a simple deflation process. State indexes were developed by aggregating weather indexes for individual locations. When the weather index is used to deflate the effect of weather on yields, the actual yield series may be adjusted to show the technological yield trend without the effects of weather. Variation in the adjusted yield series is an estimate of the effect of changes in technology. The weather index was also used to facilitate the analysis of the distribution of weather effects and the effect that improved technology has had in reducing fluctuations due to weather.