Ok, my real problem is bigger than this, but I have a simple working example.

```
>>> import pandas as pd
>>> import numpy as np
>>> a = pd.DataFrame(np.array([[2, 1990], [4,1991], [5,1992]]), \
...                  index=[1,2,3], columns=['var', 'yr'])
>>> a
   var    yr
1    2  1990
2    4  1991
3    5  1992
>>> b = pd.DataFrame(index=a.index, columns=['new_var'])
>>> b
  new_var
1     NaN
2     NaN
3     NaN
>>> b[a.yr<1992].loc[:, 'new_var'] = a[a.yr<1992].loc[:, 'var']
>>> b
  new_var
1     NaN
2     NaN
3     NaN

```

I desire the following output:

```
>>> b
  new_var
1       2
2       4
3     NaN

```