Proxy variables are used to estimate productivity in businesses. This study compares different methods for estimating productivity in Spanish manufacturing firms. The researchers found that the choice of estimator has a bigger impact on productivity measures than the choice of proxy variables. They also discovered that the assumption that productivity increases with the proxy variable is not always true. Additionally, different estimators can lead to different results when comparing productivity between exporters and non-exporters. Timing assumptions and choosing the right estimator are crucial for accurate productivity estimates.