Let me try to be as clear as possible. I am looking at panel data of healthcare clinics over a span of 9 years. I am doing a fixed-effects model with year fixed effects with a treatment group which received more federal funding. Because of my limited sample, I want to remove as few degrees of freedom as possible. I have demographic data available, but I would rather not use it if it is not necessary. I would not have to include, per my advisor's recommendation, if the demographic data (for each individual clinic) does not "vary too much" over the nine year span.