class_docstr: Todo:
  check treatment of const, make it optional ?
      add hasconst (0 or 1), needed when selecting nfact+hasconst
  options are arguments in calc_factors, should be more public instead
  cross-validation is slow for large number of observations
code: def fit_fixed_nfact(self, nfact):
      if not hasattr(self, "factors_wconst"):
          self.calc_factors()
      return sm.OLS(self.endog, self.factors[:, : nfact + 1]).fit()
  
file_path: statsmodels/statsmodels/sandbox/datarich/factormodels.py
imports: import numpy
  import statsmodels.api
  from statsmodels.sandbox.tools import pca
  from statsmodels.sandbox.tools.cross_val import LeaveOneOut
label_desc: Use image node_id 3 for calling the FactorModelUnivariate obj's underlying member method code with example usage: obj.fit_fixed_nfact(nfact) and returns: sm