I know some sources say 1 million, but I can't argue that they are more reliable.
Figures that I quoted seem to be more reliable as they originally come from Franz Halder's War Diary and Oberkommando des Heeres documents.
====================================================================================
It it is possible to establish estimated value of each army's "relative performance" against the German army.
This "relative performance" doesn't say us which army performed better, but it says us which army performed better under certain circumstances and with certain ratio of forces (which is not equal to simple ratio of manpower). Thus it says us which army would probably perform better under same circumstances and with same forces against ditto strong enemy forces.
This "relative performance" (combat effectiveness in proportion to available resources versus the available resources of the enemy) is called the Combat Effectiveness Value - it's more or less the ability to produce fighting power out of available manpower taking into account available resources and circumstances.
This is called CEV ("Combat Effectiveness Value").
There is also the "Relative Personnel Effectiveness Equivalence Value" (EV), which is - contrary to CEV - not a factor in measuring "pure" combat efficiency, more of a measure of per capita military productivity. It's because EV doesn't take into account such things like firepower gap, technological gap, ratio of available amount of weapons and their lethality values. That's why the army with larger quantity of equipment per capita and more lethal and more modern equipment will always have a better EV than the opposite army, unless it has a very poor CEV compared to the opposite army.
Measuring "combat effectiveness value" is somehow complicated:
From:
http://smartech.gatech.edu/bitstream/1853/5439/1/weerasinghe_chalinda_d_200312_ms.pdf
Chapter 4:
Page 50, definition 1:
"Available Combat Potential (ACP) is the latent capacity of a force to achieve useful results in combat with its existing organization, training, equipment, support, motivation, and leadership."
Page 51, definition 3:
"Combat Power (CP) is the realized capability of a force at any instant of time to achieve results in combat in furtherance of a particular mission against a specific enemy force in a specific combat environment."
Pages 57 - 58:
CP = S * V; where:
S = Force Strength, it is being established basing on the Operational Lethality Index (OLI); S = "the sum of the OLIs for the weapons inventories, after having been modified by these weapons effects variables (variable factors that determine the effectiveness of weapons, such as weather, terrain, season, ammunition supplies, etc.)".
-------------------------------------------------------------------------------------------------------------------------
As you can see S is not being established basing on the number of men involved but basing on the number of weapons of each category and type involved and taking into account OLIs of these weapons.
Counting men would only be correct if both forces had same weapons in same amounts with same OLIs.
----------------------------------------------------------------------------------------------------------------------
V = variables affecting the employment of the force under the circumstance existing at the time of the battle of engagement; "those variable factors affecting the force as a whole, rather than just the weapons."
It is necessary to establish the exact CP of a certain force first, to be able to establish its CEV later:
Page 54, definition 5:
"The Combat Effectiveness Value (CEV) of a relative force in a certain engagement is defined as the ratio of the relative results of that battle for that force versus the other force at that given engagement, over the ratio of the relative CPs of that force versus the other force at that given engagement."
Page 58:
There are also 3 criteria of combat effectiveness:
1) Mission effectiveness
2) Spatial effectiveness
3) Casualty effectiveness
All of them taken together denote R (The Result Value). R = "the ratio of the actual battle outcomes."
Pages 59 - 62:
Many variables comprise the CEV (Combat Effectiveness Value).
Among these variables that comprise the CEV are for example (these are considered most important):
1) Operational Variables:
a) Factor Values not established:
- Logistical Capability
- Intelligence Service
- Command and Control
- Communications
- Momentum
- Initiative
- Time and Space
- Chance
- Friction
- [maybe also Luck ?]
b) Factor Values established:
- Posture and Fortifications
- Mobility
- Vulnerability
- Air Superiority
- Surprise
- Fatigue
- Weapon Sophistication
- [what about Military Doctrine ?]
- [what about Military Strategy ?]
- [what about Military Tactics ?]
2) Environmental (all Factor Values established):
- Terrain
- Weather
- Season
- Day/Night
3) Behavioral (all Factor Values not established):
- Leadership
- Training
- Experience
- Morale
- Manpower Quality
- Third Party Intervention ("stab in the back" or something like this)