 When you do statistics with SOFA, you need to know the level of measurement for your variables in order to choose the proper statistical test. The lowest level of measurement is the nominal or categorical level. For example, if you want to find out someone's favorite color of t-shirt, you ask them to choose a category, yellow, green, or blue. If you assign numbers to those categories, they are arbitrary. You can't say that yellow is somehow less than green, and you certainly can't do any meaningful arithmetic with the numbers. So, at the categorical level, the numbers are just labels. The next level of data is the ordinal level. You have ordinal data when you only know the rank order of the values. For example, let's say you know the order in which three people finished a race. The only thing you can say for sure is that the person in first place did better than the person in second place, who did better than the person in third place. You can't do meaningful arithmetic with the numbers. Third place isn't equal to first plus second. Here's another example of ordinal data. Let's say two people rate a movie on a scale of 1 to 10. You can put them in rank order. You know the person who rated the movie at 10 likes the movie more than the person who rated it at 3. But you still can't do meaningful arithmetic with the numbers. The person who rated the movie at 10 doesn't have seven units of liking more than the other person. For ordinal data, all you can determine is the order. Who likes it more and who likes it less? The next level of measurement is interval data. Consider two people with different heights. Not only can you tell the rank order, the 176 centimeter person is taller than the 170 centimeter person, the difference or interval between the numbers is meaningful. You can do meaningful arithmetic with the values for height. 176 centimeters equals 170 centimeters plus the 6 centimeter difference or interval. Another example of interval data is the number of pairs of shoes a person owns. Not only can you tell that the person with three pairs of shoes has more than the person with two pairs of shoes, the difference is mathematically meaningful. So, to sum up the levels of measurement. In the nominal level, numbers are just labels for categories. At the ordinal level, the numbers tell you only the rank order. And at the interval level, the interval or difference between values is mathematically meaningful.