Skill decrement possible? (Anti-skilling interaction)

I'm editing an interaction on a custom object that builds skill. I would like for it to instead decrease the skill at the same rate (e.g. -1 multiplier). Is this possible? If so, what would I need to add?

<V t="dynamic_skill">
<U n="dynamic_skill">
<U n="skill_loot_data_override">
<E n="effectiveness">StandardPeriodic</E>
<T n="stat">16659<!--skill_Fitness--></T>
</U>
</U>
</V>

I'm using version 3.2.3.0

Thanks