Growth Curves for Sitka Spruce Trees in 1989
Sitka89 data frame has 632 rows and 4 columns. It gives repeated
measurements on the log-size of 79 Sitka spruce trees, 54 of which
were grown in ozone-enriched chambers and 25 were controls. The size
was measured eight times in 1989, at roughly monthly intervals.
P. J. Diggle, K.-Y. Liang and S. L. Zeger (1994) Analysis of Longitudinal Data. Clarendon Press, Oxford