76 Reading: Calculating Percentage Changes and Growth Rates

Photo of a cross-section of a tree trunk, revealing its growth rings.

In order to measure elasticity, we need to calculate percentage change, also known as a growth rate. The formula for computing a growth rate is straightforward:

[latex]\text{Percentage change}=\frac{\text{Change in quantity}}{\text{Quantity}}\\[/latex]

Suppose that a job pays $10 per hour. At some point, the individual doing the job is given a $2-per-hour raise. The percentage change (or growth rate) in pay is [latex]\frac{\$2}{\$10}=0.20\text{ or }20\%\\[/latex].

Now, recall that we defined elasticity as the percentage change in something divided by the percentage change in something else. Let’s take the price elasticity of demand as an example. The price elasticity of demand is defined as the percentage change in quantity demanded divided by the percentage change in price:

[latex]\text{Price elasticity of demand}=\frac{\text{Percentage change in quantity demanded}}{\text{Percentage change in price}}\\[/latex]

There are two general methods for calculating elasticities: the point elasticity approach and the midpoint (or arc) elasticity approach. Elasticity looks at the percentage change in quantity demanded divided by the percentage change in price, but which quantity and which price should be the denominator in the percentage calculation? The point approach uses the initial price and initial quantity to measure percent change. This makes the math easier, but the more accurate approach is the midpoint approach, which uses the average price and average quantity over the price and quantity change. (These are the price and quantity halfway between the initial point and the final point.   Let’s compare the two approaches.

Suppose the quantity demanded of a product was 100 at one point on the demand curve, and then it moved to 103 at another point. The growth rate, or percentage change in quantity demanded, would be the change in quantity demanded [latex]{(103-100)}[/latex] divided by the average of the two quantities demanded [latex]\frac{(103+100)}{2}\\[/latex]. In other words, the growth rate:


Note that if we used the point approach, the calculation would be:

[latex]\frac{(103–100)}{100}=3\%\text{ growth}\\[/latex]

This produces nearly the same result as the slightly more complicated midpoint method (3% vs. 2.96%). If you need a rough approximation, use the point method. If you need accuracy, use the midpoint method.

In this module you will often be asked to calculate the percentage change in the quantity, but keep in mind that this is the growth rate. That way as you work through the course and find a need to calculate the growth rates beyond elasticity, you will be well prepared.


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