MedStats Google Group

Re: {MEDSTATS} Re: Trajectory modelling, useful with only two timepoints?

srmil...@yahoo.com (SR Millis) — Fri, 06 Nov 2009 22:59:36 UT

Thomas,

What I mean by response bias is whether an individual is responding to your symptom questionnaire in a reliable and valid manner. Most symptom questionnaires do not have any internal scales or indices to determine this. By contrast, of the personality assessment scales, the MMPI-2 and the recent MMPI-2-RF have several scales assessing validity:

Re: Population Attributable Risk (PAR) from Cross Sectional Studies?

mca...@yahoo.com (mcap) — Fri, 06 Nov 2009 21:19:55 UT

While I can see how the idea of population attributable risk is
appealing in terms of translating results, it is still only a
numerical calculation based on study data. As such there are several
issues.

Cross sectional studies, as you indicate are inadequate for causal
inference because of temporality. So I would not use one to calculate

Population Attributable Risk (PAR) from Cross Sectional Studies?

jeam...@gmail.com (Shaibu) — Fri, 06 Nov 2009 12:58:50 UT

Hello Forumers:

I have a question. I am trying to wrap my head around the calculation
and use of Population Attributable Risk based on a cross sectional
study.
From my basic epidemiology courses, I have come to understand that
such an estimate is only reasonable when one can make plausible
assumptions of causality. Given that cross sectional studies are

Re: {MEDSTATS} Re: confidence interval calculation

m861h...@btinternet.com (Martin Holt) — Thu, 05 Nov 2009 18:57:09 UT

Hi Ellen,

I'd like to refer you to this link:

[link]

and to this link

[link]

Here it says that logistic regression and indirect standardisation are equivalent only with a fully saturated logistic model, that is with all main effects and all interactions....a bit much with 8 independent variables. The second link discusses the pros/cons of removal of (interaction) terms from the model. And whether using logistic regression is better than the 'manual' method.

Re: {MEDSTATS} Re: Trajectory modelling, useful with only two timepoints?

jeremy.mi...@gmail.com (Jeremy Miles) — Thu, 05 Nov 2009 17:09:28 UT

Hi Adrian
Proc Traj can be thought of as a special case of a latent class model.
(Specifically, if you do a latent class model with groups defined on
the slopes and intercepts, and you fix the variances of the latent
variables within groups to zero, you've got a proc traj model).
It's sort of zero inflated Poisson, but it's not really because the

Re: {MEDSTATS} Re: Trajectory modelling, useful with only two timepoints?

peterflomconsult...@mindspring.com (Peter Flom) — Thu, 05 Nov 2009 14:19:00 UT

Thomas Fröjd <tfr...@gmail.com> wrote
OK, now you've got 6 scores (3 at each time point, instead of 1). With this, there
are a lot more possibilities.
Again, I'd start with graphs .... x axis would be time, y axis score, and a line for each person
on each pair of measures. Whether this can be done on one graph depends on N. If you have more than

Re: {MEDSTATS} Re: Trajectory modelling, useful with only two timepoints?

peterflomconsult...@mindspring.com (Peter Flom) — Thu, 05 Nov 2009 13:53:23 UT

These are known by other names, too, such as "lie scales"; the intent is to
detect people who are giving incorrect answers on purpose, to appear "good" or "bad".

The best known of these is, I think, the Crowne-Marlowe (sp?) scale. They often ask respondents to respond to
statements that very few people could honestly endorse ....

Re: {MEDSTATS} Re: Trajectory modelling, useful with only two timepoints?

tfr...@gmail.com (Thomas Fröjd) — Thu, 05 Nov 2009 13:31:18 UT

Hi SR Millis

We have been using the IES-R (R for revised) scale, the 22 questions
version.

I must admit I don't really know what a response bias scale is. The
things I have read about response bias mostly takes it up in the
context of survey design and how to design questions. As I understand

Re: {MEDSTATS} Re: Ratios as DVs and IVs: BMI (was Re: Interestingresults, how to deal with these interaction effects?)

bwea...@lakeheadu.ca (Bruce Weaver) — Thu, 05 Nov 2009 13:23:32 UT

Hi John. I have no poison arrows. But, in the examples of BMI's
usefulness you provide above, it is used as an explanatory variable.
Kronmal addresses this situation in his article, and argues pretty
persuasively (I think) that one is better off including height and
weight (and possibly their product) as terms in the model. One

RE: {MEDSTATS} Re: Trajectory modelling, useful with only two ti

ted.hard...@manchester.ac.uk (Ted Harding) — Thu, 05 Nov 2009 13:17:12 UT

That's nothing! (And it's quite easily done). Once upon a time
it was the only way to do such things (in the absence of a pen
plotter, which was cutting-edge technology at the time).
People are very used to things like Tukey's box-plot which was
first developed back in the 1960's & 70's when all that most people

Re: {MEDSTATS} Re: Trajectory modelling, useful with only two timepoints?

srmil...@yahoo.com (SR Millis) — Thu, 05 Nov 2009 12:45:57 UT

I'm coming late to this discussion, so this question may have already been answered. Does your PTSD scale have an response bias scale to rule out symptoms over endsorsement or under endorsement? PTSD is quite easy to "fake" on essentially every PTSD scale I've ever encountered.
SR Millis

Re: {MEDSTATS} Re: Trajectory modelling, useful with only two timepoints?

tfr...@gmail.com (Thomas Fröjd) — Thu, 05 Nov 2009 12:29:33 UT

There are subscales. Intrusion, avoidance and hyperarousal. Looking
for patterns in change between the individuals sounds like a great
idea. Any ideas about how this could be done?

On 5 Nov, 12:36, Peter Flom <peterflomconsult...@mindsprin g.com>
wrote:

Re: {MEDSTATS} Re: Trajectory modelling, useful with only two timepoints?

adriansay...@gmail.com (Adrian Sayers) — Thu, 05 Nov 2009 12:21:17 UT

I wonder if you have thought about latent class modeling. I dont know
a great deal about it, but it sounds like it may be the thing your
after. as it look at patterns within data and how they change over
time.

From the guys i know that use this, they all do it in m-plus.

I am no expert, but might be worth a look

Re: {MEDSTATS} Re: Trajectory modelling, useful with only two timepoints?

peterflomconsult...@mindspring.com (Peter Flom) — Thu, 05 Nov 2009 11:36:41 UT

Thomas Fröjd <tfr...@gmail.com> wrote
You're certainly welcome! Interesting discussion, and I'm learning, too.
That makes sense.
Now we get into "what the data show" vs. "what theory says". I know a bit about PTSD, and I don't think "low" and "high" adequately describe the possibilities. For one thing, there are subscales to PTSD. Do you have subscale scores?

Re: {MEDSTATS} Trajectory modelling, useful with only two timepoints?

tfr...@gmail.com (Thomas Fröjd) — Thu, 05 Nov 2009 10:52:36 UT

Hi, Thank you all for your timely replies.

Let me clearify the reson behind the analysis first. As Peter and Jeff
pointed out, two measures are not really a repeated measures study and
not enough to model growth properly. This data is from the first
follow up in a cohort study and will in the future be extended with