Double Shifts & Indeterminate Outcomes

You’ll be able to

Predict the effect on price and quantity when both curves shift
Identify which of price and quantity is indeterminate in each double-shift case
Explain what additional information resolves an indeterminate outcome

When both curves move, one outcome is always ambiguous

Shift one curve and both price and quantity are determined. Shift both, and exactly one of the two becomes indeterminate — its direction depends on the relative sizes of the shifts, which the question has not told you. Recognizing that ambiguity is the skill being tested, and writing "indeterminate" is the correct answer rather than a failure to decide.

The four double-shift cases

D right + S right → Q ↑, P indeterminate · D left + S left → Q ↓, P indeterminate · D right + S left → P ↑, Q indeterminate · D left + S right → P ↓, Q indeterminate

Curves moving the SAME direction determine quantity. Curves moving in OPPOSITE directions determine price.

Why that pattern holds

When both curves shift the same way, both push quantity the same direction, so quantity is settled — but they push price opposite ways, so price is not. When they shift opposite ways, both push price the same direction, so price is settled and quantity is not. You do not need to memorize four cases; you need to see which variable both shifts agree about.

What resolves it

The indeterminate outcome becomes determinate once you know which shift is larger. If a question says demand rose "slightly" while supply rose "substantially", that is the extra information: the supply effect dominates, so price falls. Exam questions do sometimes supply this, and when they do the answer is no longer indeterminate — so read for magnitude language before concluding.

Worked example

A new technology lowers the cost of producing laptops at the same time that consumer incomes rise. Determine the effect on price and quantity.

1.Cheaper production shifts supply right.
2.Higher income shifts demand right, since laptops are a normal good.
3.Both shifts raise quantity, so quantity unambiguously rises.
4.Supply right lowers price, demand right raises it — so price is indeterminate.

Answer: Quantity rises; the price change is indeterminate without knowing which shift is larger. If the technology effect dominates, price falls; if the income effect dominates, price rises.

Watch out

"Indeterminate" is a real answer, and refusing to give it costs points. But it is only correct for the one variable that is genuinely ambiguous — claiming both are indeterminate is wrong, because one of them always is determined.

Checkpoint

Demand shifts right while supply shifts left. The result is:

Checkpoint

A drought destroys part of a wheat crop while a health report reduces demand for bread. The effect on the wheat market is:

Checkpoint

Supply increases substantially while demand increases only slightly. Price will:

On the exam

Draw both shifts and then check each variable separately: do the two arrows agree about price? About quantity? The one they agree about is determined, the other is not. This is faster and safer than recalling four memorized cases.

Answer the 3 checkpoints as you read.