Crops > Cost & Return > Yields

Collection of county yield data, how does NASS do it?

AgDM Newsletter
April 2013

The 2012 average corn and soybean yields for counties and districts in Iowa were released in late February 2013. This information is collected by USDA’s National Agricultural Statistics Service (NASS) each year using the December Agricultural Survey and County Agricultural Production Survey. Through the Ag Decision Maker website, we provide this data in Information Files A1-12 and A1-13, Historical Yields by County, which show county averages from 2003 through 2012. This information is helpful for seeing trends in yields over the past 10 years. Information File A1-14, Iowa Corn and Soybean Yields, also shows the 10-year average yield, and the year and yield results for the highest and lowest years for each county in the past 10 years. This information is helpful in developing corn and soybean budgets, cash-flow projections or other types of analysis for producers in which the actual production history is not available. The crop yields are reported in bushels per harvested acre; some programs such as Average Crop Revenue Election (ACRE) use bushels per planted acre.

NASS conducts the December Agricultural Survey (DAS) each year to establish state and national estimates of row crops such as corn and soybeans. The County Agricultural Production Survey is also conducted each year to collect data that are combined with the DAS data and used to establish the county level yields. Each year, a combined 15,000 randomly selected operators in Iowa are interviewed for these surveys. The operator reports the whole farm’s planted and harvested acreage, yield and production for corn, soybeans and hay. They also are asked to report acres rented from someone else. Other crops such as wheat and oats are collected earlier in the year.

The data are collected using several methods: mail, telephone interview, personal interview or the operator can even report electronically. Data collection begins in late fall and continues through mid-January. Trained enumerators or census takers collect the data. The same enumerators are used to collect data for NASS year-round. Strict guidelines are followed in all states to ensure comparable results on a national level.

Several steps are taken to verify the accuracy of the reports. The first step is a check for reasonableness, and any questionable results are double-checked with the operator. The results are then entered into a secure computer system and checked again for extreme yields and outliers in the data. At this point, the data are ready to be analyzed. NASS uses a system called Interactive Data Analysis System or IDAS. With this program, they can graphically look at all data that has been reported. It can be broken down by district and county at this point as well. During this phase, outliers are once again identified but by district and county. These are checked once more with the operator for accuracy.

The data are then summarized by district and county (or point estimates) for acreage planted and harvested, as well as yield. The summary indications are compared against "administrative data" from the Farm Service Agency (FSA) and Risk Management Agency (RMA) at the county level. Established estimates are reviewed by the NASS Agricultural Statistics Board in Washington, D.C. This board reviews Iowa estimates as well as other states to check for consistency and once again for accuracy. After this final review, the acreage and yield estimates are published and made available online.

Summary information is available on the Ag Decision Maker website. For other county estimates, including other crops, livestock and farm numbers, visit the NASS website for Iowa at:


Ann M. Johanns, extension program specialist, 641-732-5574,