A survey conducted on 20 households in a locality by a group of students resulted in the following frequency table for the number of family members in a household: | Family size | 1 - 3 | 3 - 5 | 5 - 7 | 7 - 9 | 9 - 11 | | Number of families | 7 | 8 | 2 | 2 | 1 | Find the mode of this data.