ˈiːðɚɹ ɪz kˈælkjʊlˌeɪɾᵻd baɪ tˈeɪkɪŋ ðə ɹˈeɪʃɪˌoʊ ʌvðə vˈæljuː ˈæftɚ ðə sˈɜːfɪs ɔːɹ ˈɛlɪmənt tə ðə vˈæljuː bᵻfˈoːɹ.
Either is calculated by taking the ratio of the value after the surface or element to the value before.