Wargame Thoughts and Commentary
Creative Wargaming

IMP, Occam's Razor, and PBN


When we design historical war-games we are attempting to reflect some aspects of decision making and model physical actions that real commanders and troops demonstrated during battles. We read a wide variety of sources, often make notes about the events we read about, and we frequently find contradictory information, incomplete accounts, and degrees of variance in the stated outcome of events. The description of the decision of certain general can be highly subjective, and it is not uncommon to find huge gaps and omissions in the description of events. How do we weigh this information? How do we decide what or who to believe? What tools can we use to sort the wheat from the chaff, or the gold from the dross?

The two that I have found are Inherent Military Probability (IMP) and Occam’s Razor.

Inherent Military Probability was first proposed as a tool by Arthur Higgins Burne, an ex-officer in the Royal Artillery, and later, a military historian who authored several books on ancient , medieval, and early gunpowder warfare. (He co-wrote a book on the ECW with Peter Young). HIs original premise was that as you tried to decipher historical evidence and accounts you should apply a test of “What would a trained staff officer of the 20th century most likely have done?” If it wouldn’t make sense to him-it probably didn’t happen that way. If it does make sense then it had to be more strongly credited. This was later amended by some to say that it must make sense to a person in that era as some situations may not have a direct historical corollary to the modern mind.


It remains mildly controversial, and has had some singular successes and failures as an approach, but I think it is an invaluable tool when used correctly. Correct use requires really thinking a account or report through and examining it logically. Ultimately the question is “Does this make sense?” as important as the provenance of the remark, its source, or the authority of the account. It requires judgement and knowledge.

An example of my first use of this in wargaming many years ago is a series of articles I wrote in the old Courier about the use of artillery. At that time in the 70s, many gamers used “Ricochet Sticks” to denote where a ball ricocheted “Over” a unit and had no effect, and where in was low enough to have effect. I thought about this a long time and turned to elementary physics and ballistics to prove that this didn’t make sense. A ball fired from a smoothbore gun at zero elevation will NEVER rise higher than the gun muzzle, and every ricochet will be below the height of a man. If fired at a higher angle their will be fewer ricochets (remember skipping a rock on a water surface?)if any, and they will all be lower than a man, or a man on horse back. In certain extreme cases of terrain, where the target is on the backslope of a hill, or the ball hits say the top of a stone wall, it may fly over a man, but almost certainly will bury in on its next impact. In effect, the IMP of a ricochet clearing a man height is very low and ricochet sticks are representing a nonexistent factor. There was a great kerfluffle by the advocates of this equipment until General B.P. Hughes book, “Firepower” came out about a year later stating the exact same finding. Ricochet sticks disappeared from the wargame table a victim of IMP and physics!

Another such finding in my articles was that during the era of Smoothbore Artillery, heavier weight guns had more effect on a single infantry or cavalry target than lighter guns when firing ball. This was easily dispatched as nonsense when logic and physics was again applied. The size difference in diameter between 4-6-8-and 12 pound field artillery balls was not very much-less than an inch in diameter for all but the 4 lb. and only an inch and a half for that!. That is, the area of effect was nearly identical! (remember they non-explosive rounds) So all artillery hard shot should have equivalent effect on a single target. Where they varied was MASS which made the heavier guns able to penetrate through many more units before the ball’s motion was arrested. They were, for the same reason more impactful on solid objects such as walls, fortifications, etc. They also had a much higher effectiveness with their canister, and a somewhat longer theoretical range, but that was seldom of great use. What they did not have was a higher effect with roundshot on a single unit to their front. IMP-QED!

The above examples are easy manifestations using physics and math, but using an IMP based on your general military reading as to the likeliness of certain behavior bolstered by a general view of people in real life and their reaction to stress and conflict is an invaluable tool for assessing information to be used in a design.

The other tool is Occam’s Razor. This premise was set forward by William of Occam in the 14th century as a means of judging the most logical explanation for a single event. In its simplest form it merely states that, when faced with several explanations or causes for an event, always look the simplest, least involved, and uncomplicated explanation-always. One way to phrase this is when you hear hoof-beats behind you always think of horses approaching, not zebras!


Now, this does not preclude complex answers, or scientific data, but simply says the simplest explanation with the fewest assumptions is a good place to start. This is a conspiracy killer, an answer to Rube Goldberg explanations for events and long involved explanations of why Junior missed school yesterday. The use of Occam’s razor for a war gamer designing rules, or deciding the accuracy of unverifiable reports, is a very handy tool. Go for the clean and simple. Eschew obfuscation!

Finally, I’d like to comment on rule users themselves, and how they effect their own enjoyment of rules. I am open to argument on this at my Yahoo! site, but it strikes me that we have seen a lessening of experimentation and creative growth in rule users, more than in rule designers! I can’t remember a time when, when playing a set of rules I didn’t think of a better way to do something within that rule structure, or come up with a new extension of the rules, or redefine some aspect of their use. Usually I did this to better suit my idea of how things occurred in a period battle, or to make the rules more playable-FOR ME. This also led to my answering a lot of my own questions about certain points of rules that might be unclear- I seldom asked the designer, unless I was impossibly confused. If I liked the core principles of a set of rules, I was more than happy to be creative in order to make them even better for my use!

That appears to be less common among war gamers now. In effect they want their war-games to be the equivalent of oil paintings done by the number. They want to be told where to put a color. They want to be told the exact shade of that color. They want firm lines denoting exactly where the boundaries between colors are. They want to be told the exact and precise nature of the image being created. Tell me the color. Tell me the number. Show me the finished picture.

Mona Lisa

Even worse, they never even consider different shades and hues, or how to actually paint, but just want to be told what to do. They want all their answers supplied, and they want no responsibility to figure it out or experiment on their own.

What is needed for a great wargame is the gamer must be a creative painter, and throw away the numbered canvas. He must try to grasp the inherent principles of a design which are usually fairly easy to grasp after a reading or two, and a couple of times on the table, but then he should take it on himself to innovate, to try new ideas, and bend the rules to his liking. He should use a set of rules as a base for his own creativity and exploration of history. He should really try to become his own artist rather than always going to the Master for interpretation and certification.

Part of this may stem from the fantasy backgrounds of many current gamers, where there is no real world to use as a touchstone for their ideas, but only the limited universe found between the covers of a 128 page, full glossy, Codex. The gamers using these rules are looking to fit in to a group rather than to strike out creatively and as an individual. They are also constrained by the corporate game publisher’s restricting their ideas to “Official Rules” and “Official Figures.” That is fine for an adolescent, but an adult in a creative hobby should at least try to be creative and an individual and not just a passive recipient of some imaginary construct.

Historical gamers have no such excuse. They are the heirs, as I have stated before, of creative writers such as Stevenson, Wells, Pratt, and Featherstone, and their hobby begs for added entertaining narrative that may include a light hearted comment on the human condition. They are missing so much, if they don’t give up painting by the numbers, and learn to paint!