Pavel Kroupa discusses the variability of the Initial Mass Function (IMF) in star-forming regions. He defines an average or Galactic-field IMF, noting that changes in the power-law index occur at two masses: near 0.5 $M_{\odot}$ and 0.08 $M_{\odot}$. Kroupa investigates the uncertainty in observational estimates of the IMF by studying the scatter introduced by Poisson noise and the dynamical evolution of star clusters. The results show that the apparent scatter in the power-law index is consistent with the observed scatter, defining a fundamental limit within which any true variation becomes undetectable. The absence of evidence for a variable IMF suggests that any true variation in well-studied populations must be smaller than this scatter. Kroupa also quantifies systematic errors in determining the power-law index $\alpha$ due to unresolved binaries, finding that young star clusters have a systematically steeper IMF compared to the Galactic-field IMF. He discusses the implications of these findings for the IMF in globular clusters and ancient populations, suggesting that higher-metallicity environments produce relatively more low-mass stars. The study concludes that the quest for detecting variations in the IMF has not yet found conclusive evidence, but theoretical arguments support the expectation of systematic variations.Pavel Kroupa discusses the variability of the Initial Mass Function (IMF) in star-forming regions. He defines an average or Galactic-field IMF, noting that changes in the power-law index occur at two masses: near 0.5 $M_{\odot}$ and 0.08 $M_{\odot}$. Kroupa investigates the uncertainty in observational estimates of the IMF by studying the scatter introduced by Poisson noise and the dynamical evolution of star clusters. The results show that the apparent scatter in the power-law index is consistent with the observed scatter, defining a fundamental limit within which any true variation becomes undetectable. The absence of evidence for a variable IMF suggests that any true variation in well-studied populations must be smaller than this scatter. Kroupa also quantifies systematic errors in determining the power-law index $\alpha$ due to unresolved binaries, finding that young star clusters have a systematically steeper IMF compared to the Galactic-field IMF. He discusses the implications of these findings for the IMF in globular clusters and ancient populations, suggesting that higher-metallicity environments produce relatively more low-mass stars. The study concludes that the quest for detecting variations in the IMF has not yet found conclusive evidence, but theoretical arguments support the expectation of systematic variations.