How a Swap’s Performance Changes Its Portfolio Weight

Summary

The document asks how to update the weight of a long or short swap after its value changes. It distinguishes a holding’s value relative to the portfolio’s original value from its weight in a portfolio whose total value has also changed. A long position with a positive return is used to illustrate that the holding’s marked value, measured against the original portfolio value, rises in proportion to its performance.

The unresolved case is a short swap whose value falls: the question proposes dividing its negative initial weight by a performance multiplier and asks for a mathematical explanation. No answer is included, so the document does not establish a formula for the short position or explain how to normalize by the current portfolio total. Its useful lesson is the accounting problem it raises: a signed exposure, the market value of a position, and a portfolio weight are related but distinct quantities. Calculating a current weight requires knowing the position’s current value and the denominator used for the portfolio.

Key ideas

• A holding’s value relative to original portfolio value changes with its performance multiplier.
• The document asks how to update a negatively weighted short swap after its value declines.
• It does not provide a resolution or derive the proposed calculation.
• A current portfolio weight depends on both the position’s current value and the portfolio-value denominator.

Tags

# How does negative performance of a portfolio constituent affect its weight?


# How does negative performance of a portfolio constituent affect its weight?












This is an easy question, I hope.

Suppose we have a swap A with a long position, which, originally, has a weight of 30%. Over time, it has a positive performance of 3%, meaning we have a multiplier of 1.03 relative to its original value. Hence the resulting weight would be 30% * 1.03 = 30.9% (as a % over original portfolio value). This makes sense, because you expect the value of the holding in A to increase as its performance increases, hence so will the weight.

If we now have a swap B with a short position with an original weight of -20%, and the swap performance is -2% (0.98 relative to original value), we would expect the portfolio to perform better as you benefit from the drop in value of the swap.

How would I work out the resulting weight as % of the portfolio's original value? Is it simply a case of -20/0.98 = -20.4%? A mathematical explanation would be appreciated.

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This summary was written by Stratmill's research agent from the original; it is not a copy of the source.