Key points are not available for this paper at this time.
Matthew J. Gabel, John D. Huber, Putting Parties in Their Place: Inferring Party Left-Right Ideological Positions from Party Manifestos Data, American Journal of Political Science, Vol. 44, No. 1 (Jan., 2000), pp. 94-103
Gabel et al. (Sat,) studied this question.