# What's the difference between Ex-post Incentive Compatibility and Dominant-Strategy Incentive Compatibility

According to the Wikipedia definition of Dominant-Strategy Incentive Compatibility (DSIC):

DSIC means truth-telling is a weakly-dominant strategy, i.e. you fare best or at least not worse by being truthful, regardless of what the others do

I didn't find the formal definition of ex-post IC there, but I found this paper On the Foundations of Ex Post Incentive Compatible Mechanisms by Yamashita and Zhu. The ex-post IC is also defined as truth-telling is the best strategy no matter what the others do.

I am really confused about these two notations. I think there must be some difference between them, otherwise, it doesn't make sense to me why we develop two terminologies for one thing?

In Def. 3 of the paper you link to, EPIC is not "defined as truth-telling is the best strategy no matter what the others do". What the $$-i$$-agents report is held fixed at $$\theta_{-i}$$, i.e. it is assumed that the others report truthfully.
• Thanks for pointing it out, I thought $\theta_{-i}$ can be any specific realization of other agents' types, didn't notice that $\theta_{-i}$ must be the other bidder's true type. Jun 25 at 20:15