Approximating the Likelihood Function of CMB Experiments

We discuss the problem of constraining cosmological parameters with cosmic microwave background band--power estimates. Because these latter are variances, they do not have gaussian distribution functions and, hence, the standard $\chi^2$--approach is not strictly applicable. A general purpose approximation to experimental band--power likelihood functions is proposed, which requires only limited experimental details. Comparison with the full likelihood function calculated for several experiments shows that the approximation works well.