I've been working through market share exercises trying to determine how much growth/decline in one supplier's results impacts the overall blended share from one period to another.

Let's say we have anywhere from 13-16 tracked suppliers for whom we calculate our market share compared to competitors. Our share with one supplier changes each quarter based on sales over total available market for a period. I then take a weighted average to determine what our overall share position is with all 13-16 tracked suppliers.

A crude example:

supplier A: our sales grew 5% while that supplier's available market declined 5% ... we gained 100bps of share (let's say went from 15% to 16% QoQ)
supplier B: our sales declined 10% while that supplier's available market declined 12% ... we gained 20bps of share because our sales decelerated more slowly than the market did
supplier C: our sales declined 6% while the supplier's market grew 10% ... we lost 150bps of share
Overall share of the 13 suppliers combined went from 19.4% to 18.4% (a drop of 100bps overall)

I'm trying to figure out how much of the overall 100bp drop was due to supplier C dropping 150bps or supplier A gaining 100bps?

I don't know if this is best addressed by a regression tool or some kind of 2-step rate/volume exercise. Any thoughts?