Calculation of sex- and age-standardized growth metrics using the LMS method (lambda-mu-sigma). The package includes functions for the CDC Growth Charts (cdc_z) and the WHO Charts (who_z). Because CDC recommends using the WHO Charts for children under 24 months and the CDC Charts among older children, there can be large differences at age 2.0 years. For example, a girl weighing 9.9 kg would be at the WHO 10th percentile on the day before her second birthday, but at the CDC 2nd percentile the following day. The 'gradual_z' function reduces the differences among 2- to 5-year-olds by taking a weighted average of the CDC and WHO z-scores.