The selection effect measures how the securities or positions chosen within a category performed relative to that category's benchmark return.
It is a core component of Brinson Attribution and is designed to separate within-category selection from the decision to overweight or underweight the category.
Common formula
For category i, a common three-effect Brinson formula is:
Selection_i = wb,i × (Rp,i - Rb,i)
where:
wb,iis the benchmark weight in the category;Rp,iis the portfolio's return within the category; andRb,iis the benchmark category return.
Using benchmark weight helps isolate relative performance within the category from the portfolio's active category weight.
Example
Assume:
- benchmark sector weight: 25%;
- portfolio sector return: 10%; and
- benchmark sector return: 8%.
Then:
Selection = 25% × (10% - 8%) = 0.50%
The positive 50-basis-point effect indicates that the portfolio's holdings within that sector outperformed the sector benchmark under the selected attribution convention.
Selection versus allocation
Allocation Effect asks whether active category weights helped or hurt relative performance.
Selection asks whether the portfolio's holdings within the category beat or lagged that category's benchmark return.
Interaction Effect captures the joint impact when an active category weight and within-category relative performance occur together.
Positive selection is not automatically stock-picking skill
A positive selection effect is an attribution result for a realized period. It does not by itself prove persistent security-selection skill.
The result can be affected by concentration, factor exposures, benchmark composition, transaction timing, incomplete holdings data, or chance.
A manager-skill assessment requires broader evidence than a single attribution effect.
Methodology still matters
Attribution systems can differ in category definitions, weight conventions, interaction treatment, multi-period linking, derivative handling, and benchmark design.
The same portfolio can therefore show different selection effects under different valid methodologies.
The most useful interpretation starts by understanding what decision the attribution model is actually trying to isolate.
Sources
- CFA Institute, Portfolio Performance Evaluation, 2026
- CFA Institute, 2026 CIPM Level I Topic Outline
- CFA Institute Research Foundation, Performance Attribution
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