Units of acreage of potatoes: Acres vs. hectares

Visit any data series page in FRED and you'll see a "Notes" section below the graph. The metadata there provide details that help tell the story behind the numbers. Arguably, the single most important metadata are the data units.

Our FRED graph above shows potato farming acreage in four different countries-the US, France, Germany, and Great Britain-between 1829 and 1951.

Just as languages can differ across countries, so can the units of measure. So when using data to measure the land area used to plant potatoes, potatos, Kartoffeln, or pommes de terre, it's important to confirm all data series are using the same units.

In this case, data from continental Europe were reported using the decimal system (in hectares) and data from Great Britain and the US were reported using the imperial system (in acres). Fortunately, FRED makes data customization very easy: Use the "Edit Line" tab to select the relevant series that use hectares and input the formula a*2.47105 to effectively convert those units to acres.

After customizing the potato acreage data, we can properly compare the disparate size of potato farming across these four countries. Note the very large land area that Germany devoted to potatoes. If we hadn't accounted for the difference in units, we could have easily drawn a completely different conclusion. What a dis-tater that could have been.

A note about the metadata in FRED's notes section:

• Source. Name of the organization reporting the data.
• Release. Name of the publication in which the data are reported.
• Units. Standardized quantities used to measure a concept.
• Notes. Additional information about the methods used to report the data or guidelines for sharing those data.
• Suggested citation. Bibliographical citation of the data series shown in the graph, ready to copy and paste .

How this graph was created: Search FRED for and select "Potato Crop Acreage for United States." From the "Edit Graph" panel, use the "Add Line" tab to search for and select "Potato Crop Acreage for France." Click on "Add data series." Next, use the "Edit Lines" tab to select "Line 2" and customize the data by typing the formula "a*2.47105" and clicking "Apply." Repeat the "Add data series" steps to search for and add "Potato Crop Acreage for England, Wales, Scotland, Great Britain" and "Potato Crop Acreage for Germany." Lastly, customize the data for Germany applying the formula spelled out above.

Suggested by Diego Mendez-Carbajo.