def join_population_data(daily_data, population_data):
    """
    Merges daily_data and population_data dataframes
    Parameters
    ----------
    daily_data : df 
        dataframe of daily observation
    population_data : df 
        dataframe of population    
    Returns
    -------
    merged df
        merged dataframe from daily_data and population_data
    """
    return daily_data.merge(population_data, how = 'left', on = 'Country/Region')