The word 'Additional' means 'in addition to other clues you discover', i.e. it modifies the number of clues that you would find, it does not add an extra effect on top of any other effects (e.g. does not stack with Burglary).
- If you commit Deduction to another player's investigation attempt, that other player would discover 1 additional clue, not you.
This bad boy doesn't have a review? Well lets remedy that!
This is one of those skill cards that tick the boxes for good skill cards. "This skill card gives a numerical bonus on a test and":
- Furthers the board state.
- Improves my character state.
Generally cards that don't tick the boxes above tend not to be quite as good as the rest. For example the reason many many decks choose Guts is because it has a high change at "healing" damage or horror, as well as gaining you a card.
Deduction ticks the former box, "Furthers board state". You gain 2 clues from a location for 1 action, taking you 1 step closer to the endgoal, not to mention that it helps you get the original clue with it's +1. Action compression is one of the most useful mechanics a card can grant, Deduction does this all for free, making it one of the best skills in the game and a go-to card to fill out the deck of anybody who can bring it in-faction, whether you're Joe Diamond or Daisy Walker.
Note that the card is a drastically weaker pick in solo, due to the prevalence of single clue locations, but in this case it is still a great card as a means to finish 2-clue spots more easily, especially if its a big 3+ shroud victory location, definitely still worth a slot..