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© 2014 Pacific Crest
429
M
ethodology
W
hat
-I
f
M
odeling
Scenario:
You wish to determine how much paint to buy in order to paint a room in your
home.
Step
Explanation
Watch it Work!
1.
What is likely to change?
Determine which inputs
have a significant likelihood
of changing and which will
remain fixed.
The number of coats of paint and
surface area are not certain
2.
How much will they
change?
Considering all probable
scenarios, what values may
the inputs take on?
The room will need between 1
and 3 coats of paint. A maximum
estimate of the area is found by
assuming all the walls are painted,
while a minimum subtracts the
area of the windows, trim, and
fixtures.
3.
Test the inputs one at a
time
For each input identified in
step 1, calculate the model
output for the range of values
found in step 2.
Fixing the number of coats of paint
to 2, we find the volume of paint
needed based on the range of
areas. Then we vary the number
of coats to see the effect in the
amount of paint needed.
4.
What effects do the
changes have?
Identify the relationship
between changes in inputs
and outputs and verify that
they agree with empirical
data and the intended model
results.
Increasing the either of the inputs
leads to an increase in the amount
of paint needed. The amounts
agree with estimates given on the
paint can.
5.
Which input has the most
impact?
Identify and rank the input
variations leading to a
significant change in the
output.
The different estimates for area
only lead to a different number of
cans needed in the case where
3 coats are used. The number of
coats is always significant.
6.
Analyze the results
Analyze the results from
step 3 and use the model to
determine the total uncertainty
in the output.
The values produced by the model
indicate that between 1 and 4
gallons are needed to paint the
room.
7.
Interpret your findings
Interpret the uncertainty
and impact analysis in the
context of the problem you
are modeling, providing a
recommendation for further
modeling.
The what-if modeling suggests
that we buy 4 gallons but keep our
receipt in case we need to return
any unused paint.
9.3 What-If Modeling