Intelligence & Skill points in Fallout: New Vegas

Intelligence increase the rate you get spendable skill points per level, but what i am wondering is this retroactive?

The calculation = 10 + (0.5 * Intelligence ) per level.

So a level 10 character with 1 Intelligence would have 15 skill points

And a level 10 character with 2 Intelligence would have 20 skill points

But there are ways of increasing the Intelligence stat in game.

So what would happen if @ level 5 the player goes from 1 Intelligence to 2 Intelligence. Would the end result be 17.5, or will it be retroactively added up and become 20.

(So basically, is the calculation about how many skill points gained, or how many skill points a character should have at that level)

No, this is not retroactive.

As such, it is best to try and reach the final level of Intelligence that you want as early as possible, to maximise the number of Skill Points that you gain.

Notably, only Permanent Intelligence increases will increase the Skill Points gained. There are only 3 of these that can be used for Intelligence:

• The Intelligence Implant, from the New Vegas Medical Clinic (Other Implants also exist)
• The Intense Training Perk (May be taken up to 10 times, +1 to any SPECIAL stat)
• Completing the "Lonesome Road" DLC (+1 to any SPECIAL stat)

The only one of these that can be gained with minimal levelling is the Implant — you can sneak/run all the way from Goodsprings to the Medical Centre without any combat. However, it costs 4,000 caps.

Any other increases or decreases to Intelligence are ignored when calculating the number of Stat points gained at level-up.