1) Keep in mind that your target sample is not “girls aged 15-19 years” but “sexually active girls aged 15-19 years” which are identified by conducting individual interviews with girls aged 15-19 years. Therefore, the number of interviewed “girls aged 15-19 years” must be high enough so that even if some are discounted due to not being sexually active, the resulting number of interviewed sexually active girls still meets the minimum sample size.
2) It is important that you understand the main limitation of the proposed methodology - it only measures the proportion of sexually active girls using contraception. It does not consider whether the respondent is currently pregnant, trying to conceive a baby, and other reasons for not using contraception. If you want to assess the proportion of girls who currently do not wish to become pregnant but who are not using a contraceptive method (i.e. the "unmet need for family planning"), you will have to use a much more complex methodology (read guidance at Measure Evaluation's website).
3) Consider including additional question assessing the main reasons for not using contraception. If the answer to Q1 is "no", you can ask:
Q: What is the main reason why you did not use a contraceptive method?
A: (multiple answers possible)
1) was pregnant
2) wanted to get pregnant
3) did not agree with using contraception (for religious or other reason)
4) her partner did not agree with using contraception
5) she thought that her partner would not agree with using contraception
6) someone else in her family / community disagreed with her using contraception
7) did not know any contraceptive method
8) did not know where to purchase contraception
9) felt too shy or affraid to go and purchase contraception
10) the contraceptive method she normally uses was not available
11) forgot contraception at home / elsewhere
12) did not have money for purchasing contraception
13) was affraid of contraception's side effects
14) was exclusively breastfeeding
15) other reason - specify: ...........................
4) This topic can, in many contexts, be very sensitive – consider therefore only having women as your interviewers. Ensure that no one can hear the respondent's answers.
5) For a useful guide on the monitoring and evaluation of adolescent reproductive health programs (including a wealth of additional indicators), read Pathfinder International’s Guide to Monitoring and Evaluating Adolescent Reproductive Health Programs (see below).