# Factor or ordered factor?

I don't understand when I should transform a qualitative ordinal independent variable into an unordered factor and when I should transform it into an ordered factor instead, when I'm performing a regression in R. For example, let's consider these explanatory variables:

• day of the week (Monday,..., Sunday): this is a qualitative variable. It is also ordinal because: Monday < Tuesday < ... < Sunday.
• month: this is a qualitative variable. It is also ordinal because: January < February < ... < December.
• year: this is a qualitative variable. But it is also ordinal because there's a natural ordering between years.
• education (with levels "Low","Medium","High"): this is a qualitative variable. It is also ordinal because: Low < Medium < ... < High.

From a statistical point of view, these variables are all qualitative ordinal, however sometimes they are treated as unordered factors in R. Why? Is there a general rule which lets me know when I should treat a qualitative ordinal explanatory variable as an ordered factor and when I shouldn't in a regression problem? Thank you in advance.

• Month and year are certainly not qualitative... You can add, subtract, multiply, divide and do all the kind of mathematical operations on years (comparing to only looking at their order as in ordinal variables) and the results have a precise numerical meaning.
– Tim
Oct 3, 2015 at 16:46