  Low, Lewis and Prescott (1999) showed that a crossover design based on a
Williams Latin square of order 4 can suffer substantial loss of efficiency if
some observations in the final period are unavailable. Indeed, if all
observations are missing, the design becomes disconnected. We derive the
information matrix for the direct effects of a Uniformly Balanced Repeated
Measurements Design (UBRMD) in t periods when subjects may drop out before the
end of the study and examine the maximum loss of information. The special case
of loss of observations in the final period only is examined in detail. In
particular we show that a UBRMD in t>= 5 periods remains connected when some or
all observations in the final period are unavailable.
