Science wouldn't be science without the facts, research, and numbers to back up all those theories. Can you imagine Albert Einstein trying to promote his famous theory of relativity without any numbers, research, or the famous formula E=MC²?

The hard numbers behind any good research project are called quantitative data. Quantitative data is the language of science. It uses mathematical models, theories, and hypotheses.

Thanks to this type of data, we know things like the distance between the sun and the moon, and why we have seasons, days, and nights.

Quantitative data depends on the fact that you can put everything in terms of numbers. "This hot chocolate is hotter than that hot chocolate" might be a true statement. But it is not as informative as "This hot chocolate is 100 degrees Fahrenheit; therefore, it is 30 degrees hotter than this other cup of hot chocolate, which is 70 degrees Fahrenheit."

Quantitative data and qualitative data, in which you observe the non-numerical qualities of your subject, go hand-in-hand. Measurements are very important to science and observation. But knowing and testing how something feels and smells is also important. In fact, knowing how something feels and smells may lead you to wonder why it feels and smells that way. Before you know it, you're using numbers and formulas to find your answer.

A frequent traveler might make a qualitative observation that the Hartsfield-Jackson Atlanta International Airport, in the U.S. state of Georgia, is very busy.

The quantitative data about passenger traffic at Hartsfield-Jackson would prove that the airport is, in fact, the busiest in the world: In 2008, more than 90 million passengers used Hartsfield-Jackson.

Scientists determined this by comparing quantitative data from other airports. The next-busiest airport was Chicago O'Hare International Airport, in the U.S. state of Illinois, which handled 69.4 million passengers. The busiest airport outside the United States was Heathrow Airport in London, England. It handled 67 million passengers. Each of the airports provides that quantitative data, also known as numerical data, to the public.