In the following APA formatted reference, what does the 21 refer to?
Harris, R. M., Bausell, R. B., Scott, D. E.,
Hetherington, S. E., & Kavanagh, K. H. (1998). An intervention for changing high-risk HIV behaviors of African American drug-dependent women. Research in Nursing and Health, 21(3), pp. 239-250.
1. Volume number
2. Issue number
3. Chapter number
4. Bibliographic reference