  The DAMA collaboration reported an annually modulated rate with a phase
compatible with a Dark Matter induced signal. We point out that a slowly
varying rate can bias or even simulate an annual modulation if data are
analyzed in terms of residuals computed by subtracting approximately yearly
averages starting from a fixed date, rather than a background continuous in
time. In the most extreme case, the amplitude and phase of the annual
modulation reported by DAMA could be alternatively interpreted as a decennial
growth of the rate. This possibility appears mildly disfavoured by a detailed
study of the available data, but cannot be safely excluded. In general, a
decreasing or increasing rate could partially reduce or enhance a true annual
modulation, respectively. The issue could be clarified by looking at the full
time-dependence of the DAMA total rate, not explicitly published so far.
