function mse = expdecaymse(t, x, tau)
% Mean squared error for a decaying exponential.
%
% Arguments: t, vector of time points.
%            x, vector of the corresponding measured data values.
%            tau, value for the time constant. 
%
% Returns:   mse: mean squared error
  mse = mean((x - expdecay(t, tau)).^2);
end