By "long" data, I mean datasets that have massive historical sweep — taking you from the dawn of civilization to the present day. The kinds of datasets you see in Michael Kremer’s "Population growth and technological change: one million BC to 1990," which provides an economic model tied to the world’s population data for a million years; or in Tertius Chandler’s Four Thousand Years of Urban Growth, which contains an exhaustive dataset of city populations over millennia.

Long data is an effective tool even when dealing with current or future phenomenon…If we don't look at these events from a historical standpoint…we have the potential to fall prey to the shifting baseline syndrome. When this happens, we regard events happening in our current decade to be the norm and are often blinded to what has happened in generations past. This is why it is important to add long data to our toolkit—because it gives us context.

What exactly is big data depends on who you ask. Some people talk about lots of measurements (what I call "fat data"), others of huge numbers of records ("long data"), and some talk of both.