The study compares two ways to measure how easily inputs can be swapped in a production process. One method, called the Antonelli elasticity, looks at how inputs can be substituted while keeping output the same. The other method, called the Hicks elasticity, assumes that costs stay the same when inputs are swapped. The study finds that the Antonelli method gives a more accurate measure of input substitution than the Hicks method. The two methods are shown to be the same when the production process has constant returns to scale. The study also provides a way to estimate the Antonelli elasticity from data on how much of each input is used.