The column iid appears to be the composition of two numbers and the first number looks like a business id. Part 1.: Create a new column called bid in the ins dataframe containing just the business id. You will want to use ins['iid'].str operations to do this. Also be sure to convert the type of this column to int Part 2.: Then compute how many values in this new column are invalid business ids (i. e. do not appear in the bus['bid'] column). This is verifying a foreign key relationship. Consider using the pd. Series. isin function