Vendor diversification index

The vendor diversification index is a single score that measures how evenly a business spreads its spend across the suppliers in a category. It takes the spend share of every vendor, squares each share, sums them, and subtracts that sum from one. The result sits between zero and almost one. A category served entirely by one vendor scores near zero, because there is no diversification at all. A category split evenly across many vendors scores close to one.

The squaring is what makes the index useful. By squaring each share before summing, the calculation gives extra weight to large, dominant vendors. A category where one vendor holds 90 percent scores far lower than one where the largest holds 30 percent, even if both have the same number of suppliers on paper. The index rewards genuine balance, not merely a long supplier list with one giant inside it.

The vendor diversification index is the mirror image of vendor concentration risk. Concentration measures how exposed you are to your largest vendor; diversification measures how well that exposure is spread. Tracking the index lets you compare categories on a single scale and see at a glance which ones rest dangerously on one supplier and which are comfortably balanced.

The index is built from a small set of inputs. The calculation itself is straightforward, but each input shapes how meaningful the resulting score is, so it is worth understanding what feeds in before you read the number.

1. 1

   Vendor spend shares

   For each vendor in the category, divide its spend by total category spend. These shares must sum to one and form the raw material of the index. Use a consistent period, such as a rolling twelve months, so the shares are stable.

2. 2

   Sum of squared shares

   Square each vendor share and add them together. This sum is the concentration component. A single vendor at full share gives a sum of one, while many even vendors give a small sum, and the squaring penalises dominance heavily.

3. 3

   The index itself

   Subtract the sum of squared shares from one. The result runs from zero, meaning total concentration in one vendor, up towards one, meaning highly diversified. This is the headline figure you track and compare across categories.

4. 4

   Effective vendor count

   Optionally, divide one by the sum of squared shares to get the effective number of vendors. A category with one dominant and several tiny suppliers might have eight names but an effective count near two, which is far more honest than the raw count.

A worked example makes it concrete. Suppose a category has three vendors holding 60, 30, and 10 percent. The squared shares are 0.36, 0.09, and 0.01, summing to 0.46. The index is 1 minus 0.46, which gives 0.54. Now split the same spend evenly across three vendors at one third each. The squared shares sum to about 0.33, so the index rises to 0.67. The same three vendors produce a meaningfully more resilient score once the dominance of the largest is removed.

A single index score tells you a category is concentrated. It does not tell you which vendor causes it or which lever would fix it. A metric tree decomposes the overall vendor diversification index into the categories, vendors, and structural factors that drive it, turning one score into a set of targeted procurement moves.

The first level splits the company-wide index into the index of each spend category, because a healthy average can hide one badly concentrated category dragging beneath the surface. Each category branch then decomposes into the spend share of its dominant vendor, the effective vendor count, and the share of spend that has a qualified alternative ready. The tree shows precisely where a single supplier is pulling the score down and how far from balance that category sits.

KPI Tree models this by attaching RACI ownership to every branch and pushing the change to the accountable owner when a category index drops below its threshold. The category is Decision Intelligence, and the gap it closes is the one between a dashboard reporting a diversification score and a decision to award the next contract to a second vendor. When the category owner can see exactly which supplier is causing the concentration, rebalancing spend becomes a deliberate, owned action rather than a number nobody acts on.

The right index depends on the category, since some inputs are genuinely best sourced from a single trusted vendor while others demand a spread. The ranges below give a practical reading of the score for a critical category where resilience matters. Treat them as a guide for where to look, not a target to chase everywhere.

Resist the urge to drive every category to the top of the scale. Beyond a point, extra diversification costs more in management and lost buying power than the resilience it buys. A critical category sitting comfortably in the 0.5 to 0.7 band, with a qualified backup ready, is usually in a healthier position than one pushed to 0.9 across a sprawl of tiny suppliers nobody can manage well. Read the index alongside the criticality of the category, not in isolation.

Improving the index means deliberately moving spend away from a dominant vendor towards a credible alternative in the categories that warrant it. The work is less about adding suppliers and more about building a second source you can actually rely on, then shifting real volume to it.

1. 1

   Counting vendors instead of weighting spend

   Ten vendors mean nothing if one holds 90 percent of spend. The index exists precisely to penalise that dominance through squaring. Always weight by spend share, never by headcount of suppliers.

2. 2

   Chasing a high index everywhere

   Maximising diversification in every category adds cost and weakens buying power for little gain. Apply the index where resilience matters and accept healthy concentration where a single trusted vendor is genuinely best.

3. 3

   Averaging across categories and missing the weak one

   A strong company-wide index can hide one critical category resting on a single vendor. Always read the index per category, and watch the lowest-scoring critical one most closely.

4. 4

   Treating an unqualified alternative as diversification

   Splitting a contract on paper does not diversify supply if the second vendor cannot actually deliver. The index only reflects real resilience once the alternative has handled meaningful volume.