Answer: It is aklways assume to be a positive number: energy must be added to initiate the reaction, but will be sustained by the heat of it's own reaction.
Explanation: I never thought about this, but my answer would be that, by it's nature, activiation energy is always supplied to the system, so it is assume to be positive, unlike enthaly, which is either added (endothermic, positive) or released (exothermic, negative) from the system. A negative activation energy makes no sense - it only tells us the reaction may be spontaneous.
