{"id":23,"date":"2001-10-21T00:00:39","date_gmt":"2001-10-21T07:00:39","guid":{"rendered":"http:\/\/www.mii.ucla.edu\/causality\/?p=34"},"modified":"2001-10-21T00:00:39","modified_gmt":"2001-10-21T07:00:39","slug":"indirect-effects","status":"publish","type":"post","link":"https:\/\/causality.cs.ucla.edu\/blog\/index.php\/2001\/10\/21\/indirect-effects\/","title":{"rendered":"Indirect Effects"},"content":{"rendered":"

From <\/strong>Melanie Wall, University of Minnesota:<\/strong><\/font><\/p>\n

I am teaching a course in latent variable modeling (to biostatistics and other public health students) and was yesterday introducing path analysis concepts including direct and indirect effects. <\/font><\/p>\n

I showed them how to calculate indirect effects by taking the product of direct paths. Then a student asked about how to interpret the indirect effect and I gave the answer that I always give, that the indirect effect ab<\/em> (in the following simple model) is the effect that a change in x<\/em> has on Z<\/em> through its relationship with Y<\/em>.
<\/font><\/p>\n

After chewing on this for a second, the student asked the following: <\/font><\/p>\n

Student:<\/strong> "The interpretation of the b<\/em> path is: b<\/em> is the increase we would see in Z<\/em> given a unit increase in Y<\/em> while holding X<\/em> fixed, right?" <\/font><\/p>\n

Me:<\/strong> "That's right" <\/font><\/p>\n

Student:<\/strong> "Then what is being held constant when we interpret an indirect effect?" <\/font><\/p>\n

Me:<\/strong> "Not sure what you mean" <\/font><\/p>\n

Student:<\/strong> "You said the interpretation of the indirect effect ab<\/em> is: ab<\/em> is the increase we would see in Z<\/em> given a one unit increase in X<\/em> through its causal effect on Y<\/em>. But since b<\/em> (the direct effect from Y<\/em> to Z<\/em>) requires X<\/em> to be held constant how can it be used in a calculation that is also requiring X<\/em> to change one unit" <\/font><\/p>\n

Me:<\/strong> "Hmm. Very good question, I'm not sure I have a good answer for you. In the case where the direct path from X<\/em> Z<\/em> is zero I think we have no problem since the relationship between Y<\/em> and Z<\/em> then has nothing to do with X<\/em>. But you are right, here if "c<\/em>" is non-zero then we must interpret b<\/em> as the effect of Y<\/em> on Z<\/em> when X<\/em> is held constant. I understand that this sounds like it conflicts with the interpretation of the ab<\/em> indirect effect where we are examining what a change in X<\/em> will cause. How about I get back to you. As I have told you before, the calculations here aren't hard, its trying to truly understand what your model means that's hard." <\/font><\/p>\n

<\/font><\/p>\n","protected":false},"excerpt":{"rendered":"

From Melanie Wall, University of Minnesota: I am teaching a course in latent variable modeling (to biostatistics and other public health students) and was yesterday introducing path analysis concepts including direct and indirect effects. I showed them how to calculate indirect effects by taking the product of direct paths. Then a student asked about how […]<\/p>\n","protected":false},"author":3,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[19],"tags":[],"class_list":["post-23","post","type-post","status-publish","format-standard","hentry","category-indirect-effects"],"_links":{"self":[{"href":"https:\/\/causality.cs.ucla.edu\/blog\/index.php\/wp-json\/wp\/v2\/posts\/23","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/causality.cs.ucla.edu\/blog\/index.php\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/causality.cs.ucla.edu\/blog\/index.php\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/causality.cs.ucla.edu\/blog\/index.php\/wp-json\/wp\/v2\/users\/3"}],"replies":[{"embeddable":true,"href":"https:\/\/causality.cs.ucla.edu\/blog\/index.php\/wp-json\/wp\/v2\/comments?post=23"}],"version-history":[{"count":0,"href":"https:\/\/causality.cs.ucla.edu\/blog\/index.php\/wp-json\/wp\/v2\/posts\/23\/revisions"}],"wp:attachment":[{"href":"https:\/\/causality.cs.ucla.edu\/blog\/index.php\/wp-json\/wp\/v2\/media?parent=23"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/causality.cs.ucla.edu\/blog\/index.php\/wp-json\/wp\/v2\/categories?post=23"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/causality.cs.ucla.edu\/blog\/index.php\/wp-json\/wp\/v2\/tags?post=23"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}