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Risk Measurement • 255
Assume a supplier delivers $1 million worth of parts to a company in the
third quarter of a year. e supplier also commits three infractions that quar-
ter—a late delivery, missing documentation, and a parts shortage. Further
assume the buyer assigns $30,000 in total nonconformance charges to these
infractions. e supplier’s SPI for the third quarter is 1.03, or (($1,000,000 +
$30,000)/$1,000,000). e SPI of 1.03 means the total cost of doing business
with this supplier is 3% higher than the unit price. If the unit price of a sup-
plier’s good is $127, then the estimated total cost of that item is really $130.81
($127 × 1.03). Because the SPI is a standardized metric, it allows compari-
sons between suppliers. A supplier with a higher SPI has a higher total cost
than one with a lower SPI. It is important to compare suppliers within the
same commodity to ensure “apples to apples” comparisons.
SPI Drawbacks. Although the SPI can be an eective tool, it is by no
means perfect. In fact, it has some potential drawbacks that users must
understand. First, because it is an index, the actual unit cost of an item
from a supplier is not considered directly in the SPI calculation—only the
value of the total shipments and infractions are considered. A higher sup-
plier unit cost inates the value of the shipments compared with a supplier
that has a lower unit cost, making any infractions look smaller given the
shipment value. Mathematically, this makes the SPI value lower, all else
equal, for the higher- price supplier. Let’s illustrate this with two suppliers
that ship the same number of units with the same infraction charges but
with dierent unit costs:
Supplier A Supplier B
50,000 units @ $9.00 per unit 50,000 units @ $10.50 per unit
$27,500 non conformance charges $27,500 nonconformance charges
SPI = ((50,000 × $9) + $27,500)/$450,000 SPI = ((50,000 × $10.50) + $27,500)/$525,000
= 1.06 = 1.05
e dierence here is a mathematical artifact of the dierent unit costs.
Ideally, the buying company would employ a total landed cost model dur-
ing supplier selection so any issues regarding dierences in unit costs
would have already been considered.
e SPI calculation also has a built- in bias against small volume sup-
pliers. Assume three suppliers within a commodity group commit
the same infraction that resulted in a $3,000 nonconformance charge.
e rst supplier provided $15,000 worth of goods during a quarter, the