# Correlation skew mapping

What methods can be used to map the correlation skew of a credit index on a bespoke CDO portfolio?

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The basic formula is $$c_\text{bespoke}(z) = c_\text{index}\left( z \frac{EL_\text{index}}{EL_\text{bespoke}} \right)$$ though sometimes a scale factor $f$ is included like this $$c_\text{bespoke}(z) = c_\text{index}\left( z \left( \frac{EL_\text{index}}{EL_\text{bespoke}} \right)^f \right)$$