What it means
The simple version says desired capital is a fixed multiple of output, so investment equals that multiple times the change in output. Accelerator theory keeps the relationship but accepts that firms close the gap between the capital they want and the capital they have only gradually.
That gradual adjustment is the flexible accelerator, and it is what makes the theory usable for forecasting. By estimating how much of the gap a sector closes each year, analysts produce investment forecasts that do not swing from boom to zero overnight.
In applied work the theory is written as induced investment plus replacement investment, with induced investment driven by recent changes in output and replacement driven by the existing asset base wearing out. Separating the two explains why investment has a floor even in a completely flat year.
The theory matters commercially because it drives the demand forecasts of anyone selling to businesses rather than consumers. Equipment makers, engineering firms, construction companies and industrial lenders all live on induced investment, which is the most volatile part of total spending.
Its main weaknesses are worth knowing. Capital adjustment depends on confidence, credit availability and interest rates as much as on output, so accelerator models are usually combined with a cost-of-capital measure rather than used on their own.
Analysts normally test the theory by comparing the investment it predicts with what firms actually spent, then adjusting the assumed speed of adjustment until the two line up. That fitted speed becomes the forecasting assumption, and it is usually faster in light industry than in sectors where new capacity takes years to build.
In practice
Real-world examples.
Example
A bank's credit team uses an accelerator model to forecast equipment lending. When manufacturing output growth slows from 6% to 2%, the model predicts a drop in new loan demand long before applications actually fall.
Example
An industrial fan manufacturer sets its three-year capacity plan from its customers' output forecasts multiplied by an estimated capital-output ratio. The plan is deliberately staged so that only 60% of the forecast capacity is committed up front.
Example
A government economist forecasting tax receipts separates induced investment from replacement spending. Because replacement is far steadier, the split gives a more reliable floor for capital allowance claims in a downturn.
Formula
Calculation
Investment in a period = capital-output ratio x (output this period - output last period) + replacement investment. Take a sector with a capital-output ratio of 1.5, output of $400,000,000 last year and $440,000,000 this year, and an asset base of $500,000,000 depreciating at 5% a year. The change in output is $440,000,000 - $400,000,000 = $40,000,000, so induced investment is 1.5 x $40,000,000 = $60,000,000. Replacement investment is 5% x $500,000,000 = $25,000,000, giving total investment of $60,000,000 + $25,000,000 = $85,000,000. Under a flexible accelerator where firms close only 60% of the gap within one year, induced investment becomes 60% x $60,000,000 = $36,000,000 and total investment falls to $36,000,000 + $25,000,000 = $61,000,000.Case study
Seen in the real world.
Kestrel Hydraulics, an invented and purely illustrative component supplier, sold to agricultural machinery plants and had spent a decade reacting to orders rather than forecasting them.
Its new finance director built a simple accelerator model: customer output change multiplied by a capital-output ratio of 1.2, adjusted so that only half of the implied investment landed in the first year. The model suggested that a slowdown in farm output growth from 7% to 3% would cut Kestrel's order intake by about 30%, even with customers still expanding.
Kestrel used the forecast to move 15% of its workforce onto flexible contracts and to delay a $6,000,000 press purchase by a year. When the slowdown arrived broadly as modelled, the fictional company kept its margin while two competitors cut prices, which is the illustrative payoff of thinking in changes rather than levels.
Watch out
Common mistakes.
- Treating accelerator theory and the acceleration principle as unrelated ideas. The principle is the bare relationship, and the theory is the family of models that makes it workable.
- Using a single capital-output ratio for every sector. Service businesses need very little capital per dollar of output, while utilities and heavy industry need several times as much.
- Leaving replacement investment out of a forecast. Doing so predicts zero spending in a flat year, which never happens in a real economy.
Questions
People also ask.
What is the difference between the simple and flexible accelerator?
The simple version assumes capital adjusts fully within the period, while the flexible version assumes only part of the gap is closed each year.
Does the theory work for service businesses?
Less well, because much of their capacity is people rather than machines, so hiring plays the role that capital spending plays in manufacturing.
Where does the cost of money fit in?
Interest rates and credit availability change how much of the implied investment actually happens, which is why accelerator models are usually paired with a cost-of-capital measure.
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