  An order-of-magnitude estimate for the incidence of high-amplitude delta
Scuti-type variable stars (HADS) in the delta Scuti area of the H-R diagram is
calculated. Using a model for the stellar distribution in the Milky Way, we
calculate the number of stars that are expected to fall in the delta Scuti area
of the H-R diagram within the magnitude range and sky coverage of the ROTSE
Survey for Variables I (RSV1). The incidence of the HADS phenomenon is then
obtained by comparing the number of stars calculated by the model and the
actual, observed number of HADS in the RSV1. We find that ~0.3 % of the stars
that lie in the delta Scuti area of the H-R diagram within the RSV1
observational limits exhibit the HADS phenomenon. This number is much lower
than the incidence of the low-amplitude delta Scuti stars (LADS), ~>1/3,
implying that the HADS phenomenon takes place in a very small fraction of stars
and/or its duration is very short, compared to the LADS.
