  Separating foregrounds from the signal is one of the big challenges in cosmic
microwave background (CMB) experiments. A simple way to estimate the CMB
temperature in a given pixel is to fit for the amplitudes of the CMB and the
various foreground components. The variance squared of this estimator is shown
to be equal to $[(FDF)^2\ \sigzt + \sigsh^2]$, where $\sigz$ is the variance in
the absence of foregrounds; $\sigsh$ is the variance due to the uncertainty in
the shapes of the foreground components; and FDF is the {\it foreground
degradation factor}. This one number, the FDF, gives a good indication of the
ability of a given experiment to disentangle the CMB from foreground sources. A
variety of applications relating to the planning and analyzing of experiments
is presented.
