A VARIABLE must vary!

In the simplest definition, a "variable" is something that varies or changes. It is the opposite of a constant, which does not change. In statistics, a variable is something we measure, and we end up with different measurements when we measure different people or measure the same people at different times.

"My weight first thing in the morning" is a variable that changes at least slightly pretty much every single day. I step on the scale first thing in the morning and everyday I get a somewhat different readings like 130.0, 129.7, 128.9, 131,3...etc.

"Ranking on air quality" is a variable that varies from city to city according to an environmental agency that test the air quality of the major cities in the states and rank-order them. Each of the city listed has a different ranking.

"Favorite TV show" is a variable that changes from person to person. I can ask all the neighbors on my block and I will end up with many different TV shows being the "favorite TV show".

So the name of the variable has to be "generic" enough so that it can take on different "values" or "categories" if we collect data by taking a number of measurements on that variable. We do not refer to one particular measurement or value and call it a "variable." For example, the variable is "favorite TV show" instead of "CSI", "ranking on air quality" instead of #5 rank for a certain city, "my weight in the morning" instead of "130.0". If we get the hang of this, I should not see any more of the error of describing "two variables" as male and female, or as high and low rankings. Male and female are the different categories within the same variable "gender"; all rankings are the possible values within the same variable of "ranking on air quality."

Variables like "favorite TV show" is categorical or nominal, because it contains several categories with different names (nominal) and people can be sorted into those categories. 

Variables like "ranking on air quality" is ordinal, because the different rankings are in an order. Rank 1 is higher than rank 5, which is higher than rank 15.

Variables like "my weight first thing in the morning" is a continuous variable because the scale used to measure my weight is continuous one where I can get a measurement anywhere on the scale, not prescribed by a certain categories or ranks. 

The variable type has important implications in statistics because we obviously can do more complicated mathematical operations on a continuous variable, compared to ordinal or nominal variables. Imagine taking an average across the different TV shows or figuring out the air quality difference by subtracting one city's ranking from another city's ranking! Don't ever attempt these tasks at home or at school!!