controls = ['age', 'yrs_educ', 'black', 'hispanic', 'married', 'nodegree', 'earnings1975','lab_attach']

for var in controls:
    # Define the model
    mod5 = sm.ols(formula=f'{var} ~ training + low', data=df_dummies)
    res5 = mod5.fit(cov_type='HC0') 

    # Print results
    print(res5.summary())