Can I compare two within-subjects' dependent variables while holding co-variates?
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Hello All,
I want to compare a series of dependent variables, and they are all within subjects. I already done a series of paired-sample t-test. But now, I want to hold some independent variables as co-variates. However, since these co-variates are also within-subjects, I'm not sure how can I do that... Someone told me vaguely of a way including parallel regressions and than comparing the residuals... So, can I compare two within-subjects' dependent variables while holding co-variates? If so, can it be done in SPSS? Thanks in advance!! NHR |
Re: Can I compare two within-subjects' dependent variables while holding co-variates?
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Most likely a model that uses all of the design
factors at the same time would be more appropriate. But it is
hard to say without more information about your data.
What are your variables? What level of measurement are they? How man levels are there? Do the levels represent all the possible levels for that variable or are they randomly chosen from among the possible values? What role does each variable play independent, dependent, something to be controlled for? Are co-variates measured once or at the same time as repeated DVs? For repeated measures what is the nature of the repetition? time, before after, members of a pair of group (mother - father, teacher student, litter mates,etc.), different treatment conditions? How did you select you subjects and gather your data? Art Kendall Social Research Consultants On 12/19/2011 4:48 AM, NHR wrote: ===================== To manage your subscription to SPSSX-L, send a message to [hidden email] (not to SPSSX-L), with no body text except the command. To leave the list, send the command SIGNOFF SPSSX-L For a list of commands to manage subscriptions, send the command INFO REFCARDHello All, I want to compare a series of dependent variables, and they are all within subjects. I already done a series of paired-sample t-test. But now, I want to hold some independent variables as co-variates. However, since these co-variates are also within-subjects, I'm not sure how can I do that... Someone told me vaguely of a way including parallel regressions and than comparing the residuals... So, can I compare two within-subjects' dependent variables while holding co-variates? If so, can it be done in SPSS? Thanks in advance!! NHR -- View this message in context: http://spssx-discussion.1045642.n5.nabble.com/Can-I-compare-two-within-subjects-dependent-variables-while-holding-co-variates-tp5085591p5085591.html Sent from the SPSSX Discussion mailing list archive at Nabble.com. ===================== To manage your subscription to SPSSX-L, send a message to [hidden email] (not to SPSSX-L), with no body text except the command. To leave the list, send the command SIGNOFF SPSSX-L For a list of commands to manage subscriptions, send the command INFO REFCARD
Art Kendall
Social Research Consultants |
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Hi,
Thanks a lot for your replay. My dependent variables are PTSD measures. I have 3 sub-scales and one total measure. These 4 measures refer to two different events, so each participant has 8 measures (4 for every event - 4 couples). I wanted to compare, for example the degree of avoidance symptoms between event 1 and 2, and so on, regarding each couple. Each participant has 2 marks/grads, for each measure - one for every event. But, than I wand to hold as co-variates the time duration that passed since each event. I wanted to do that cause there is a significant difference between the time duration that passed between the two events. While doing an ANOVA or MANOVA, the two co-variates (duration since each event) influence the two dependent variables, and that is no good. I thinks there is no point in holding as co-variate the duration since event 1 while looking at the symptomolgy after event 2. So, I was wondering how can I compare my dependent variables in a better way? The participants were all volunteers, who agreed to participate in a web-survey. I hope my answers are clear. Please let me know if not, and I'll be happy to answer. Thanks a lot, I really appreciate it, NHR |
Re: Can I compare two within-subjects' dependent variables while holding co-variates?
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In reply to this post by NHR
NHR,
You are going to have to restructure your data and use Mixed. Look through the documentation in the syntax reference. I think what you are describing is explicitly mentioned, although I am not sure. Gene Maguin -----Original Message----- From: SPSSX(r) Discussion [mailto:[hidden email]] On Behalf Of NHR Sent: Monday, December 19, 2011 11:22 AM To: [hidden email] Subject: Re: Can I compare two within-subjects' dependent variables while holding co-variates? Hi, Thanks a lot for your replay. My dependent variables are PTSD measures. I have 3 sub-scales and one total measure. These 4 measures refer to two different events, so each participant has 8 measures (4 for every event - 4 couples). I wanted to compare, for example the degree of avoidance symptoms between event 1 and 2, and so on, regarding each couple. Each participant has 2 marks/grads, for each measure - one for every event. But, than I wand to hold as co-variates the time duration that passed since each event. I wanted to do that cause there is a significant difference between the time duration that passed between the two events. While doing an ANOVA or MANOVA, the two co-variates (duration since each event) influence the two dependent variables, and that is no good. I thinks there is no point in holding as co-variate the duration since event 1 while looking at the symptomolgy after event 2. So, I was wondering how can I compare my dependent variables in a better way? The participants were all volunteers, who agreed to participate in a web-survey. I hope my answers are clear. Please let me know if not, and I'll be happy to answer. Thanks a lot, I really appreciate it, NHR -- View this message in context: http://spssx-discussion.1045642.n5.nabble.com/Can-I-compare-two-within-subje cts-dependent-variables-while-holding-co-variates-tp5085591p5086603.html Sent from the SPSSX Discussion mailing list archive at Nabble.com. ===================== To manage your subscription to SPSSX-L, send a message to [hidden email] (not to SPSSX-L), with no body text except the command. To leave the list, send the command SIGNOFF SPSSX-L For a list of commands to manage subscriptions, send the command INFO REFCARD ===================== To manage your subscription to SPSSX-L, send a message to [hidden email] (not to SPSSX-L), with no body text except the command. To leave the list, send the command SIGNOFF SPSSX-L For a list of commands to manage subscriptions, send the command INFO REFCARD |
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Hi Gene,
I'm sorry, but I'm quite new at this... What do you mean by syntax reference? Thanks again, NHR |
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In reply to this post by Maguin, Eugene
Hi Gene,
I'm sorry, but I'm quite new at this... What do you mean by syntax reference? Thanks again, NHR On Mon, Dec 19, 2011 at 6:55 PM, Gene Maguin [via SPSSX Discussion] <[hidden email]> wrote: NHR, |
Re: Can I compare two within-subjects' dependent variables while holding co-variates?
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In reply to this post by Maguin, Eugene
Having had extensive training/education in the use of MLM/HLM (Done through mixed modeling in SPSS and SAS), and having used it extensively in my evaluation work, I get a bit nervous when I see the potential for people to jump into this form of modeling based solely on self-teaching. The SPSS documentation is wholly inadequate in my opinion, and there are some problems with the Mixed models used in SPSS, which requires an understanding of how they work to appropriately interpret them.
Common pitfalls are a misunderstanding of how to interpret the coefficients, and to over-estimate the model, without understanding the impact that is having. I do agree with Gene that a mixed model approach (More commonly known in the literature as multi-level modeling (MLM) or Hierarchical Linear Modeling (HLM) is the way to go, but for different reasons. The data you have has an event nested within a person, and a person nested within a couple. Thought of another way (and this is modeled differently), you might say that you have a person, nested within a couple, nested within an event. The latter would mean that you believe that responses to a particular even are more likely to be similar to each other than they would between events. This can thus be explicitly modeled. In terms of how to "separate" the duration from the model, you have to make a decision. Do you want to model the events separately, which will completely eliminate the effect of one event on the other, or do you want to include the other event in the modeling of the latter. If the Latter is the case, then you can do this with interaction terms of duration and event. However, you still would have the effect of the other event and it's duration modeled in the overall tests of effects. That brings up a different issue. Are you modeling two events as separate outcome variables? Multiple dependent variables cannot be modeled in the mixed method program in SPSS or SAS, and it's actually been argued that they shouldn't be anyway. Also, SPSS generally only has linear mixed models, which may be a limiting factor for your analysis. In 19 they did add a mixed GLM approach; however I've seen little documentation on it, and only just begun using it and comparing it with the HLM package, which I trust much more. Matthew J Poes Research Data Specialist Center for Prevention Research and Development University of Illinois -----Original Message----- From: SPSSX(r) Discussion [mailto:[hidden email]] On Behalf Of Gene Maguin Sent: Monday, December 19, 2011 10:43 AM To: [hidden email] Subject: Re: Can I compare two within-subjects' dependent variables while holding co-variates? NHR, You are going to have to restructure your data and use Mixed. Look through the documentation in the syntax reference. I think what you are describing is explicitly mentioned, although I am not sure. Gene Maguin -----Original Message----- From: SPSSX(r) Discussion [mailto:[hidden email]] On Behalf Of NHR Sent: Monday, December 19, 2011 11:22 AM To: [hidden email] Subject: Re: Can I compare two within-subjects' dependent variables while holding co-variates? Hi, Thanks a lot for your replay. My dependent variables are PTSD measures. I have 3 sub-scales and one total measure. These 4 measures refer to two different events, so each participant has 8 measures (4 for every event - 4 couples). I wanted to compare, for example the degree of avoidance symptoms between event 1 and 2, and so on, regarding each couple. Each participant has 2 marks/grads, for each measure - one for every event. But, than I wand to hold as co-variates the time duration that passed since each event. I wanted to do that cause there is a significant difference between the time duration that passed between the two events. While doing an ANOVA or MANOVA, the two co-variates (duration since each event) influence the two dependent variables, and that is no good. I thinks there is no point in holding as co-variate the duration since event 1 while looking at the symptomolgy after event 2. So, I was wondering how can I compare my dependent variables in a better way? The participants were all volunteers, who agreed to participate in a web-survey. I hope my answers are clear. Please let me know if not, and I'll be happy to answer. Thanks a lot, I really appreciate it, NHR -- View this message in context: http://spssx-discussion.1045642.n5.nabble.com/Can-I-compare-two-within-subje cts-dependent-variables-while-holding-co-variates-tp5085591p5086603.html Sent from the SPSSX Discussion mailing list archive at Nabble.com. ===================== To manage your subscription to SPSSX-L, send a message to [hidden email] (not to SPSSX-L), with no body text except the command. To leave the list, send the command SIGNOFF SPSSX-L For a list of commands to manage subscriptions, send the command INFO REFCARD ===================== To manage your subscription to SPSSX-L, send a message to [hidden email] (not to SPSSX-L), with no body text except the command. To leave the list, send the command SIGNOFF SPSSX-L For a list of commands to manage subscriptions, send the command INFO REFCARD ===================== To manage your subscription to SPSSX-L, send a message to [hidden email] (not to SPSSX-L), with no body text except the command. To leave the list, send the command SIGNOFF SPSSX-L For a list of commands to manage subscriptions, send the command INFO REFCARD |
Re: Can I compare two within-subjects' dependent variables while holding co-variates?
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In reply to this post by NHR
If, as you say, you are new at this, then you certainly need to keep
the analyses as simple as possible, because you have plenty of trouble in laying out simple *hypotheses* from this information and you don't need to add the difficulty of reading the tests on obscure contrasts that are only implicit -- Be explicit about what you are contrasting. See below for further comments. > Date: Mon, 19 Dec 2011 08:21:53 -0800 > From: [hidden email] > Subject: Re: Can I compare two within-subjects' dependent variables while holding co-variates? > To: [hidden email] > > Hi, > Thanks a lot for your replay. > > My dependent variables are PTSD measures. I have 3 sub-scales and one total > measure. These 4 measures refer to two different events, so each participant > has 8 measures (4 for every event - 4 couples). Eventually you may do otherwise, but for starters, take *one* measure, like the Total, and see what sense you can make of it. So now you have one measure for two events -- and you seem (below) to be interested in the *difference* of those two. Subtract... and now you have *one* measure for an analysis. If the average is also of interest, take that average, and you have another, single, measure to look at in an analysis. > > I wanted to compare, for example the degree of avoidance symptoms between > event 1 and 2, and so on, regarding each couple. Each participant has 2 > marks/grads, for each measure - one for every event. > But, than I wand to hold as co-variates the time duration that passed since > each event. I wanted to do that cause there is a significant difference > between the time duration that passed between the two events. Here's a problem for testing duration since a traumatic event: "duration" is not going to be at all equal-interval or linear, so you will get into trouble if you try to use it as measured merely in days or weeks or months or years. Three years is not much different from 4 years, in respect to recovery, though the difference is 365 days. Ten days versus 375 days is an interval that is much longer, for the difference you would expect. Maybe you collected the intervals scored into "reasonable" categories. If not, you need to address that problem. I might apply my own "expert" opinion to create categories, if I had some idea. Otherwise, if I didn't want to throw out the detail, I would start by taking the log of the durations, and (possibly) draw in the extremes (Windsordize). For durations, sometimes the reciprocal transformation (events per year, instead of years-per-event) is more appropriate. The reciprocal transformation is stronger that the log, and thus it does more to emphasize differences that are closer to zero. > While doing an ANOVA or MANOVA, the two co-variates (duration since each > event) influence the two dependent variables, and that is no good. I thinks > there is no point in holding as co-variate the duration since event 1 while > looking at the symptomolgy after event 2. > So, I was wondering how can I compare my dependent variables in a better > way? But it does make some sense to use both covariates when the criterion is the difference. I think I see a model here with one outcome and two covariates, which is not too hard to perform or interpret. > > The participants were all volunteers, who agreed to participate in a > web-survey. > > > I hope my answers are clear. > Please let me know if not, and I'll be happy to answer. [snip] Hope this helps. -- Rich Ulrich |
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