What determines mission difficulty? [duplicate]

In XCOM: Enemy Unknown missions get more difficult as I advance in the game - at least in absolute terms. I was wondering if the mission difficulty does or does not depends on

• Elapsed time (ingame)
• Plot progress
• Research progress
• other factors

Anyone has any insight into this?

Solution 1:

I do not know the exact equation but I am currently gathering data on a normal play-through. I suspect according to my data so far that mission difficulty is strongly affected by the country's current panic level increased by the other countries panic levels on the same continent. I do not yet know if rewards increase the difficulty.

Small differences can be seen in missions under otherwise similar circumstances, for example the first abduction missions had every country at panic 1 and average continent panic at 1. Here I was given the option of an easy mission or a moderate mission, indicating some sort of random variable in the Difficulty calculation.

So far it seems that if an abduction awards a soldier then the mission will be more difficult than a similar mission under the same circumstances but it could also be random.

Solution 2:

I think the game caluculates your difficulty level based on story progress and research progress. If you don't progress with the story ethereals will appear without getting the beacon. Same thing goes the other way. If you get really far with the story you get harder enemies too. Which one goes faster i dont really know. Certain enemies are bound to certain types of missions. e.g. The Crissalid appears after your first terror mission.