These data are from Vonesh and Chinchili (1997, Linear and Nonlinear
Models for the Analysis of Repeated Measurements, Section 6.6).

Low flux dialyzers are used to treat patients with End Stage Renal
Disease (ESRD) in order to remove excess fluid and waste from their
blood. In low flux hemodialysis, the ultrafiltration rate (ml/hr) at
which fluid is removed is thought to follow a linear increasing relationship
with the transmembrane pressure (mmHg) which is applied across the dialyzer
membrane. A study was conducted to compare the ultrafiltration rate
of such dialyzers across three dialysis centers. A total of 41 dialyzers
were used. The experiment involved measuring the ultrafiltration rate
at several transmembrane pressures for each dialyzer.

There are missing data in these data, and note that the repeated 
measurement is not 'time' but instead 'pressure.'  That is, there are
several measurements taken per dialyzer, one per membrane pressure value 
as pressure is increased. Also, the pressure values are not common to all 
dialyzers. This is equivalent to the longitudinal scenario where the t_ij
values are not common for all i.

Thanks to Dr. Marie Davidian at North Carolina State University 
Department of Statistics for making this data set and its background
easily available. These may be the same data as those in:

Vonesh and Carter (1987). Efficient inference for random-coefficient growth curve models with unbalanced
data. Biometrics, 43, 617-628.