The correlation between xpm and kda is 0.94. The more xpm, the more kda, and vice versa. But some values are higher than expected, some are lower. Efficiency is a difference between the value of kda depending on xpm (blue dots) with respect to the expected value (red line).
Efficiency
Ring Of Aquila 0.3436
Bullwhip 0.1589
Pupil'S Gift 0.1191
Ex Machina 0.1112
Quickening Charm 0.1085
Mirror Shield 0.1078
Force Boots 0.1066
Ceremonial Robe 0.1042
Arcane Ring 0.1030
Gossamer Cape 0.0917
Enchanted Quiver 0.0863
Stormcrafter 0.0668
Seer Stone 0.0605
Grove Bow 0.0554
Vambrace 0.0554
Titan Sliver 0.0523
Defiant Shell 0.0498
Occult Bracelet 0.0464
Eye Of The Vizier 0.0420
Spell Prism 0.0388
Dandelion Amulet 0.0283
Dragon Scale 0.0249
Timeless Relic 0.0218
Psychic Headband 0.0209
Elven Tunic 0.0176
Fairy'S Trinket 0.0156
Seeds Of Serenity 0.0118
Arcanist'S Armor 0.0088
Cloak Of Flames 0.0011
Paladin Sword -0.0098
Mind Breaker -0.0144
Apex -0.0183
Ninja Gear -0.0199
Trusty Shovel -0.0232
Book Of Shadows -0.0248
Pirate Hat -0.0265
Stygian Desolator -0.0284
Faded Broach -0.0339
Ogre Seal Totem -0.0507
Giant'S Ring -0.0532
Telescope -0.0540
Fallen Sky -0.0586
Book Of The Dead -0.0615
Pig Pole -0.0691
Penta-Edged Sword -0.0693
Trickster Cloak -0.0890
Lance Of Pursuit -0.0931
Ascetic'S Cap -0.0982
Tumbler'S Toy -0.0983
Vampire Fangs -0.0998
Spark Of Courage -0.1174
Philosopher'S Stone -0.1213
Broom Handle -0.1371
Havoc Hammer -0.1586
Martyr'S Plate -0.1831
Duelist Gloves -0.2477