Yes, I agree with Gary. Whenever you are dropping some controls you are essentially always estimating the ATE. (That's a bit of an oversimplification but not too far off). But you could estimate the ATE using subclassification. (But right, not using the MatchIt-generated weights). You would estimate the effect separately within each subclass and then weight the subclass-specific effects by the proportion of people in that subclass (in the whole population). I think my 2010 Statistical Science paper talks more about that. And I think Zelig could implement that but I'm not sure. Liz On 2/1/12 11:42 AM, "Gary King" &lt;king(a)harvard.edu&gt; wrote: With matching, its easier to think about estimating an ATT. Then you only need to ensure that you keep all the treated units, and whatever controls are useful as matches. as a separate point, i'd drop treated units when there are no sufficiently close controls. that changes the quantity of interest, which is fine so long as you clarify what you're estimating. Gary -- Gary King - Albert J. Weatherhead III University Professor - Director, IQSS - Harvard University GKing.Harvard.edu <http://gking.harvard.edu/> - King(a)Harvard.edu <mailto:King@Harvard.edu> - @kinggary <http://twitter.com/kinggary> - 617-500-7570 - Asst 495-9271 - Fax 812-8581 On Wed, Feb 1, 2012 at 11:38 AM, Francois Maurice &lt;maurice.francois(a)ymail.com&gt; wrote: Hi, I'm using MatchIt. I'm trying to understand which method produce ATE estimate and which one produce ATT estimate. In the documentation, section 5: Frequently Asked Questions: How Exactly are the Weights Created?, it is said : "These weights are constructed to estimate the average treatment effect on the treated, [...]". Is there a way with MatchIt to estimate ATE ? To be concrete, I'm using experimental data with a control group almost three times the treated group. I'm using the following four methods with ratio=2 in matchit(): Nearest : Drop some controls Subclassification : Keep all controls Nearest with exact : Drop some controls Genetic : Drop some contrls Since subclassification keeps all controls, can that be an ATE estimate or do I need to built my own weights to make sure it is ATE? In general, is MatchIt produced only match set with weights that can only be use to estimate ATT ? And if I use Zelig after MatchIt, is there a way to produce ATE estimate ? Thanks, François Maurice - --- MatchIt mailing list served by HUIT List Address: matchit(a)lists.gking.harvard.edu Subscribe/Unsubscribe: http://lists.gking.harvard.edu/mailman/listinfo/ei MatchIt Software and Documentation: http://gking.harvard.edu/matchit/ Browse/Search <http://gking.harvard.edu/matchit/Browse/Search> List Archive: http://lists.gking.harvard.edu/mailman/private/matchit/ Matchit mailing list Matchit(a)lists.gking.harvard.edu https://lists.gking.harvard.edu/mailman/listinfo/matchit