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From |
Johannes Kotte <johannes.kotte@st.ovgu.de> |

To |
statalist@hsphsun2.harvard.edu, Joerg Luedicke <joerg.luedicke@gmail.com> |

Subject |
Re: st: Modeling control variables (covariates) in SEMs: What is the correct approach? |

Date |
Mon, 21 Jan 2013 10:45:36 +0100 |

Dear Joerg,

Variables: IV ...independent variable MV ...mediator variable DV ...dependent variable CV1...control variable 1 CV2...control variable 2 *Approach A* (http://www.ats.ucla.edu/stat/stata/faq/sem_mediation.htm) is to use the control variables in each of the equations: sem (MV <- IV CV1 CV2) (DV <- MV IV CV1 CV2)

sem (IV <- CV1 CV2) (MV <- IV CV1 CV2) (DV <- MV IV CV1 CV2) Thanks for your help, everybody! Best Johannes Zitat von Joerg Luedicke <joerg.luedicke@gmail.com>:

There seems to be some confusion going on. First, what you call moderator variable rather looks like a mediator variable to me when looking at the model you show under "approach 1". Second, when we regress a on b, it means that we consider a as the outcome and b as the predictor, not the other way round. Now as for the modeling strategy, that always depends on what you want to find out etc. In data analysis, there never is just one solution for everything. That said, if you were interested in exploring indirect effects of your main predictor of interest (IV) on your outcome via the variable you call MV, then your first approach looks reasonable. From judging without knowing any context here, I would say that the second approach does not make much sense. Say you have only 3 variables, MV, IV, and DV. If you were interested in regressing DV on MV while adjusting the effect for IV, then regressing DV and MV on IV with two separate equations followed up by regressing DV on MV won't get you there. You simply end up with three separate equations (with possibly correlated errors) with which you cannot adjust the effect of MV on DV with IV. Joerg On Fri, Jan 18, 2013 at 5:29 AM, Johannes Kotte <johannes.kotte@st.ovgu.de> wrote:Hi everybody, regarding the use of control variables (covariates) in SEMs I have seen two different approaches and I am not sure which one is the correct one. Can anybody help me? Variables: IV ...independent variable MV ...moderator variable DV ...dependent variable CV1...control variable 1 CV2...control variable 2 *Approach 1* (http://www.ats.ucla.edu/stat/stata/faq/sem_mediation.htm) is to use the control variables in each of the equations: sem (MV <- IV CV1 CV2) (DV <- MV IV CV1 CV2) *Approach 2* (used by a fellow researcher I know) is to regress CV1 and CV2 on all other variables first and then do the rest of the regressions without control variables: sem (DV MV IV <- CV1 CV2) (MV <-IV) (DV<-MV) To me, this approach looks incorrect. What I find particularly confusing with this approach is that CV1 and CV2 are also being regressed on IV. I am grateful for feedback. Thanks in advance! One more question comes to my mind: Some of my CVs are correlated at the 5% level, but their correlation coefficients are below 0.3. Would you exclude these CVs? Thanks for your help! Johannes -- Johannes Kotte Otto-von-Guericke-Universität | Fakultät Wirtschaftswissenschaften | Lehrstuhl für Unternehmensführung und Organisation (Prof. Dr. Thomas Spengler) | Postfach 4120, 39016 Magdeburg | www.ufo.ovgu.de E-Mail: johannes.kotte@st.ovgu.de * * For searches and help try: * http://www.stata.com/help.cgi?search * http://www.stata.com/support/faqs/resources/statalist-faq/ * http://www.ats.ucla.edu/stat/stata/* * For searches and help try: * http://www.stata.com/help.cgi?search * http://www.stata.com/support/faqs/resources/statalist-faq/ * http://www.ats.ucla.edu/stat/stata/

-- Johannes Kotte

Telefon: +49-173-6371955 | E-Mail: johannes.kotte@st.ovgu.de * * For searches and help try: * http://www.stata.com/help.cgi?search * http://www.stata.com/support/faqs/resources/statalist-faq/ * http://www.ats.ucla.edu/stat/stata/

**References**:**st: Mediation analysis with SEM command: Why do coefficients of SEM standard output and the output of 'estat teffetcs' differ?***From:*Johannes Kotte <johannes.kotte@st.ovgu.de>

**Re: st: Mediation analysis with SEM command: Why do coefficients of SEM standard output and the output of 'estat teffetcs' differ?***From:*William Buchanan <william@williambuchanan.net>

**st: Modeling control variables (covariates) in SEMs: What is the correct approach?***From:*Johannes Kotte <johannes.kotte@st.ovgu.de>

**Re: st: Modeling control variables (covariates) in SEMs: What is the correct approach?***From:*Joerg Luedicke <joerg.luedicke@gmail.com>

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