Compassion Meditation's Efficiency In compassion meditation (metta-bhavana), does the strength of the effect depend mainly on concentration? Or are there other factors involved as well?

Thank you.
I depend on concentration and sincerity of on thoughts.

If one can keep out other distracting thoughts one's concentration increase thus increasing the effectiveness.

If one just repeating words than sincere this decreases the effectiveness.

Ultimately what is needed is:


one generates compassionate thoughts
develop compassion as a quality
break barriers where one classify beings as favourable and unfavourable