The correlation between xpm and kda is 0.88. 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
Holy Locket 0.7057
Boots Of Bearing 0.5335
Guardian Greaves 0.5263
Yasha And Kaya 0.4583
Octarine Core 0.4391
Revenant'S Brooch 0.4172
Wind Waker 0.4118
Shiva'S Guard 0.3957
Kaya And Sange 0.3954
Phylactery 0.3843
Refresher Orb 0.3604
Eye Of Skadi 0.3580
Pavise 0.3493
Gleipnir 0.2855
Solar Crest 0.2623
Disperser 0.2597
Veil Of Discord 0.2588
Spirit Vessel 0.2531
Arcane Blink 0.2440
Scythe Of Vyse 0.2385
Boots Of Travel 2 0.2368
Linken'S Sphere 0.2100
Pipe Of Insight 0.1961
Butterfly 0.1937
Rod Of Atos 0.1907
Drum Of Endurance 0.1898
Helm Of The Overlord 0.1872
Swift Blink 0.1819
Overwhelming Blink 0.1706
Vladmir'S Offering 0.1681
Meteor Hammer 0.1679
Aether Lens 0.1523
Force Staff 0.1459
Lotus Orb 0.1445
Bloodthorn 0.1383
Mekansm 0.1273
Aghanim'S Scepter 0.1242
Glimmer Cape 0.0986
Nullifier 0.0832
Manta Style 0.0830
Daedalus 0.0589
Bottle 0.0582
Magic Wand 0.0535
Crimson Guard 0.0519
Harpoon 0.0390
Eul'S Scepter Of Divinity 0.0342
Arcane Boots 0.0273
Hurricane Pike 0.0259
Dagon (5) 0.0171
Ethereal Blade 0.0034
Aeon Disk -0.0005
Heart Of Tarrasque -0.0019
Assault Cuirass -0.0064
Witch Blade -0.0071
Dagon (4) -0.0109
Radiance -0.0272
Boots Of Travel -0.0321
Moon Shard -0.0352
Null Talisman -0.0422
Aghanim'S Shard -0.0539
Tranquil Boots -0.0542
Urn Of Shadows -0.0567
Falcon Blade -0.0730
Dagon (3) -0.0738
Bloodstone -0.0810
Soul Ring -0.0839
Aghanim'S Blessing -0.0872
Abyssal Blade -0.0885
Mjollnir -0.0979
Bracer -0.1012
Orb Of Corrosion -0.1066
Dagon (2) -0.1104
Dagon -0.1271
Orchid Malevolence -0.1456
Heaven'S Halberd -0.1472
Medallion Of Courage -0.1604
Blink Dagger -0.1742
Sange And Yasha -0.1858
Phase Boots -0.2132
Desolator -0.2213
Monkey King Bar -0.2226
Eternal Shroud -0.2472
Diffusal Blade -0.2551
Power Treads -0.2741
Mage Slayer -0.2777
Satanic -0.2804
Vanguard -0.3066
Wraith Band -0.3177
Black King Bar -0.3287
Crystalys -0.3320
Echo Sabre -0.3392
Helm Of The Dominator -0.3430
Blade Mail -0.3647
Skull Basher -0.4002
Silver Edge -0.4080
Shadow Blade -0.4401
Maelstrom -0.4464
Dragon Lance -0.4514
Hand Of Midas -0.4530
Mask Of Madness -0.6079
Armlet Of Mordiggian -0.6401
Battle Fury -0.7536