def filter_patients(df):
    """
    Filter the data to contain only those patients with
    a positive effect and those treated in the last 5 years
    
    Input
    -----
    df: pandas.DataFrame
        The dataframe generated by read_data
        
    Returns
    -------
    nonzero_df: pandas.DataFrame
        The dataframe after filtering
    
    """
    nonzero_df = df[(df["Effect"] > 0) & (df["Year"] > 2015)]
    return nonzero_df