Lipsey, R. G., Olsen, W., & Forrest, D. (1989). Workbook for the seventh edition of an introduction to positive economics (Revised ed.). Oxford: Oxford University Press.

Lipsey, Richard G., Wendy Olsen, and David Forrest. Workbook for the Seventh Edition of an Introduction to Positive Economics. Revised ed. Oxford: Oxford University Press, 1989.

Lipsey, Richard G., Wendy Olsen, and David Forrest. Workbook for the Seventh Edition of an Introduction to Positive Economics. Revised ed. Oxford: Oxford University Press, 1989.