Every tech in the game has a 50% chance of not appearing in your tech tree (except for Psilons, who have only 25% chance of missing tech), unless the tech is the only available option in its "tier".
Techs in each field are grouped into tiers of 5 levels. For example, the second tier (levels 6 to 10) in Force Fields contains Personal Deflector Shield (level 8). and Class III Deflector Shields (level 10). The game shuffles the techs, then for each one flips a coin whether to remove it or not, unless it is the last tech and all the earlier techs were removed.
However, there are a few tiers which contain only one tech (or in the case of Force Fields, contains only one tech that is not in the starting set of techs). These techs are thus always assured of appearing in every game, since they are necessary to advance:
- Force Fields Tier 1: Class II Deflector Shields (Class I Deflector Shields is also in this tier, but it is a starting tech and thus does not count)
- Propulsion Tier 7: High Energy Focus
- Construction Tier 10: Neutronium Armor
- Planetology Tier 10: Complete Terraforming (a.k.a. Terraforming +120)
As you can see, two of these are very late game and hard to plan for (you are likely to reach that level only if you have essentially won the game already and are just playing with the late-game toys). Thus, there is very little scope in actually planning based on techs that are "always available", since they are so few (and two of them are so high level that the game is over by then).
However, the Class II Deflector Shields get installed onto missile bases. Since beam weapons get half damage against planets, Lasers and Gatling Lasers can still scratch missile bases that have Class I Deflector Shields, but are useless against Class II Deflector Shields. Do remember that if you built your missile bases before getting Class II Deflector Shields, you have to upgrade them!
The other always-available tech is High Energy Focus, which is a key technology that is widely considered to be the beginning of the endgame (and thus ends the defensive midgame). The midgame starts with the Class V Planetary Shield, which is not always available, but is important enough that most players will trade, invade, or spy just to get it.
Techs almost always available[edit]
Some tiers have only two techs in them, and thus both techs have a better chance of appearing in your tree (since if one is missing, the other is assured to appear in your tree). The most relevant example for planning is Propulsion Tier 1, which contains both Hydrogen Fuel Cells (Range 4) and Deuterium Fuel Cells (Range 5), and if one is missing, you will get the other. These techs might not be always available, but are common enough that you can probably plan to get one of them, if not on your actual tree, from an AI. Another example is the Class V Planetary Shield in Force Fields Tier 3, which is paired with Class IV Deflector Shield; even if you get the 37.5% chance of not having it in your tree (for a normal non-Psilon race), the chances are good that one or more of the AI races has it.
Why 37.5% instead of 50%?
Remember, the game shuffles the techs within a tier first, then starts removing them, and if the last is the only one remaining, it is assured of existing.
With two techs in a single tier, there are only two possible shufflings. Taking the Force Fields Tier 3 as an example, the possible shufflings are:
- Class V Planetary Shield, Class IV Deflector Shield.
- Class IV Deflector Shield, Class V Planetary Shield.
Each of the above has a 50% chance of occurring, because there are only two possible shufflings. Let's now consider within each possibility the probability that Planetary V is retained:
- Shuffling: Class V Planetary Shield, Class IV Deflector Shield.
- Since Planetary V is first, it has a 50% chance of existing within this shuffling.
- Shuffling: Class IV Deflector Shield, Class V Planetary Shield.
- If Class IV Deflector gets the 50% chance of being dropped, then Planetary V is assured. Thus 50% of the time, with this shuffling, Planetary V will exist.
- If Class IV does not get dropped, then Planetary V has a 50% chance of existing. Multiply this with the 50% chance of Class IV being retained, and this represents 25% of the time, with this shuffling, that Planetary V will exist.
- The sum is thus 75% of the time, with this shuffling, Planetary V will exist.
Then, multiply the 50% in the first shuffling with the 50% chance that the first shuffling is chosen, and multiply the 75% in the second shuffling with the 50% chance the second shuffling is chosen, and you get 0.5 * 0.5 + 0.75 * 0.5 = 0.25 + 0.375 = 0.625 or 62.5% chance the Planetary V will exist, or 37,5% chance the Planetary V will not exist.
Thus the quoted 50% is really an asymptote, the more techs within a tier the more accurate the 50% is, but you actually have a slightly better than 50% chance of any arbitrary tech appearing in your tree.