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
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.