What major factor prevents the simple application of star sampling methods consistently across the entire galaxy?

Answer

Star density is not uniform throughout the galaxy.

The consistency of sampling is compromised because star density varies widely; a sample from the sparse outer halo will give a result very different from a sample taken near the galactic center.

What major factor prevents the simple application of star sampling methods consistently across the entire galaxy?
measurementSpaceastronomymethodsstarsastrophysicstelescopesstar countingcounting