capitalizing names of variables in phd thesis

[marinaperegrina]

I am proofreading my Phd thesis and I don't undentand when I should capitalize the name of variables. I am using APA style. Should I capitalize them when I am talking about factors or constructs?
For instance, is this sentence correct: Letter Knowledge and Concept of Print were used as control variables.
Thanks a lot for your help
Marina

[RonBee]

I have no idea what a control variable is. Nor do I know what Letter Knowledge and Concept of Print refer to. However, if I had some context (perhaps a considerable amount of context) I might be able to come up with an opinion.

~R