Race Difficulty - Comparisons of Ultras

I came across this analysis of various races and their relative difficulty from realendurance.com. Very revealing and seems in line from my experience at San Diego and Western States. I'm about to find out about Angeles Crest 100. Three weeks and counting...

Relative Event Finish Time to Western States 100 Mile.

Sample Criteria:

Same individual completed the listed event and one of the following 100 mile run in the SAME YEAR: Western States, Vermont, Leadville, Angeles Creset, Wasatch, Kettle Moraine, Hardrock, Rocky Raccoon, and Mohican.

The relative percentage is first averaged over the samples for each 100 mile reference, then normalized to Western States, then averaged over the nine 100 mile runs listed.
The relative uncertainy of the analysis is porportional to 1/sqrt(sample size).

Time

Dist. Rel to Sample Event Name
 WS100 Size 

135m 157% 133 BadWater Ultra Marathon  
100m 155% 904 Hard Rock 100 Mile   
100m 131% 58 HURT Trail 100 Mile   
100m 117% 3279 Wasatch Front 100 Mile   
100m 113% 45 Mount Rushmore Trail 100 Mile  
100m 113% 115 Bear 100 Mile   
100m 112% 90 Bighorn Mountain Trail Run 
100m 111% 396 Massanutten Mountain Trails 100 Mile
100m 108% 14 Tahoe Rim Trail 100 miles 
100m 108% 2156 Angeles Crest 100 Mile   
100m 107% 12 Grand Teton Races 100m  
100m 106% 79 Superior Sawtooth 100 Mile 
100m 105% 80 Cascade Crest 100 Mile   
100m 103% 4508 Leadville Trail 100 Miles  
100m 100% 7497 Western States 100 Miles  
100m 99% 79 San Diego 100 Mile   
100m 96% 46 McNaughton Park Trail Runs 
100m 93% 72 Rio Del Lago 100 Mile   
100m 93% 41 Ancient Oaks 100  
100m 91% 66 Haliburton Forest 100  
100m 90% 984 Mohican Trail 100 Mile  
100m 89% 108 Javelina Jundred 100 Mile  
100m 88% 341 Old Dominion 100 Mile One Day  
100m 88% 34 SULPHUR SPRINGS 100 Mile 
100m 88% 451 Arkansas Traveller 100 Mile 
100m 88% 628 Kettle Moraine 100 mile  
100m 84% 3533 Vermont Trail 100 Miles  
100m 84% 91 Heartland 100 Mile   
100k 81% 68 HURT Trail 100k option  
100m 81% 281 Umstead 100 Mile   
100m 81% 1654 Rocky Raccoon 100m  
100m 80% 41 Old Dominion 100 mile Memorial Day
100m 76% 22 Dan Rossi Memorial Ultras 
140m 66% 17 IronMan CoeurdAlene  
140m 61% 20 IronMan Canada  
200m 60% 8 Mt Tam Double Century 
100k 58% 45 Where's Waldo 100K  
200m 57% 14 The Terrible Two  
100k 53% 97 Bandera 100km  
140m 53% 25 IronMan Wisconsin  
50m 48% 332 Zane Grey 50 Miles  
140m 47% 28 IronMan USA Lake Placid  
100k 46% 1052 Miwok 100K  
140m 46% 35 IronMan Florida  
100k 46% 42 Kettle Moraine 100k option 
140m 46% 14 IronMan Hawaii World Championship
50m 40% 99 Bishop High Sierra 50 Miles 
50m 40% 70 Tahoe Rim Trail 50 Miles 
52m 39% 340 Sierra Nevada   
100k 39% 144 Ruth Anderson 100k  
100k 39% 7 Orange Curtain 100k  
50m 37% 229 White River 50 Miles  
50m 36% 340 Silver State 50 Miles  
50m 36% 528 Quicksilver 50 Mile  
50m 36% 425 FireTrail 50 Miles  
50m 36% 80 Run on the Sly 50 Miles  
50k 35% 338 Silver State 50k  
50m 35% 342 Nugget 50 Miles   
50m 35% 19 Mt Hood PCT 50 Miles  
50m 35% 369 Leona Divide 50 Mile Run 
50m 35% 147 Umstead 50 Miles  
50m 34% 536 California 50 Miles   
35m 34% 84 Santa Barbara 9 Trails 35 Mile  
50m 33% 2568 American River 50 Miles  
50m 33% 52 McKenzie River 50 Miles  
50m 32% 414 Avalon 50 Miles  
50m 32% 41 Rocky Raccon 50 Miles  
50k 31% 203 Baldy Peak 50k  
50m 31% 163 Helen Klein 50 Miles  
50m 30% 114 Cow Mountain 50 Miles  
50k 30% 64 Tahoe Rim Trail 50k  
50m 30% 372 Jed Smith 50 Miles  
50k 28% 16 Bishop High Sierra 50 Mile and 50 Km
38m 28% 26 Haleakala Run to the Sun
50m 28% 20 Ruth Anderson 50 Miles  
50k 28% 37 Mt. Disappointment 50 Km  
50k 25% 12 Nugget 50 Miles   
50k 24% 411 Ohlone Wilderness 50K Trail Run 
50k 23% 253 Crown King Scramble 50k  
50k 23% 195 McDonald Forest 50K  
71m 23% 31 WildFlower Long Course Triathlon
50k 23% 7 Bighorn Mountain Trail Run 
50k 23% 76 Run on the Sly 50 Km  
34m 22% 36 Peterson Ridge Rumble 60k 
50k 22% 219 Golden Gate Headlands 50k 
50k 22% 58 Bandera 50k  
50k 22% 53 Bulldog 50K Ultra Run  
28m 22% 687 Quad Dipsea  
50k 21% 43 Siskiyou Out Back 50K  
50k 21% 81 Chuckanut 50k  
71m 21% 27 California Half Ironman  
50k 20% 297 Quicksilver 50K  
50k 20% 13 Ruth Anderson 50k  
50k 20% 512 Skyline 50k  
50k 20% 1392 Way Too Cool 50k  
50k 19% 73 Hagg Lake Trail Runs 50k 
50k 19% 5 Wild Wild West 50k  
50k 19% 36 McKenzie River 50k 
50k 18% 301 OTHTC High Desert Ultra 50k 
50k 17% 31 Helen Klein 50k  
50k 17% 38 Salem Lakeshore Frosty Fifty Km 
50k 17% 384 Jed Smith Ultra Classic 50K 
45m 12% 14 Santa Barbara Long Course Triathlon

A general trend can be observed below:

From 50 km to 100 mile,   factor is 4 to 6.
From 50 mile to 100 mile, factor is 2.5 to 3.5