Message-ID: <e837fef90908310953l234f885ard183182b8e5f62f@mail.gmail.com>
Date: 2009-08-31T16:53:35Z
From: Robert Terwilliger
Subject: longitudinal analysis using lmer?
In-Reply-To: <A8B693D3-9C3A-41FB-8A1A-B71D18A86E3B@kjbeath.com.au>
Thanks everyone for all the advice.
One question I have (maybe there will be more...... :-P ):
Should I exclude subjects that have only 1 or 2 data points?
On Fri, Aug 28, 2009 at 7:07 PM, Ken Beath<ken at kjbeath.com.au> wrote:
> On 29/08/2009, at 6:07 AM, Robert Terwilliger wrote:
>
>> One more thing........
>>
>> What i sent was only a small sample of the data, just for the purpose
>> of showing what kind of set we have.
>>
>> We have about 150 subjects, with starting ages between 8 and 21, with
>> 3-5 data points (yearly visits) per subject.
>>
>
> This will be fine, although it isn't as good as having a smaller number of
> complete series. One point is that they don't look completely linear, so a
> polynomial (maybe quadratic) or regression spline may be a better option.
> Judging by the scatter the random effect variance will probably be close to
> zero.
>
> Ken
>
>
>
>
>> Thanks,
>>
>> --
>> Robert Terwilliger
>> Physicist
>> Laboratory of Neurocognitive Development
>> Western Psychiatric Institute and Clinic
>> University of Pittsburgh Medical Center
>> Loeffler Building
>> 121 Meyran Avenue ?#114
>> Pittsburgh, PA 15213
>> 412.383.8174 ?- Office
>> 412.383.8179 - Fax
>> em: raterwil at gmail.com
>> http://www.wpic.pitt.edu/research/lncd/
>>
>> *******************************************************
>> Dear R mixed effects gurus,
>>
>> I have the following data below. Attached is a png graphic
>> representing the data.
>>
>> I would like to run the following analysis:
>>
>> signal ~ age | subject.
>>
>> For your information (not statically relevant), the "signal" variable
>> is from a functional MRI experiment.
>>
>> At issue is whether this analysis is valid using "lme". From the graph
>> (and the table below), one can see that there are five subjects.
>> However, each subject begins at a different age. Subject 1 begins at 8
>> and goes to 12, while subject 5 begins at 14. From my study of
>> longitudinal analysis, usually each subject begins at the same
>> starting point, while these data have subjects beginning at different
>> starting points (different ages).
>>
>> Any insight is appreciated.
>>
>> subject age ? ? signal
>> 1 ? ? ? 8 ? ? ? 0.108
>> 1 ? ? ? 9 ? ? ? 0.139
>> 1 ? ? ?10 ? ?NA
>> 1 ? ? ? 11 ? ? ?0.151
>> 1 ? ? ? 12 ? ? ?0.148
>> 2 ? ? ? 10 ? ? ?0.127
>> 2 ? ? ?11 ? ?NA
>> 2 ? ? ? 12 ? ? ?0.135
>> 2 ? ? ? 13 ? ? ?0.146
>> 3 ? ? ? 9 ? ? ? 0.105
>> 3 ? ? ? 10 ? ? ?0.123
>> 3 ? ? ? 11 ? ? ?0.134
>> 3 ? ? ? 12 ? ? ?0.151
>> 3 ? ? ? 13 ? ? ?0.145
>> 4 ? ? ? 12 ? ? ?0.130
>> 4 ? ? ? 13 ? ? ?0.169
>> 4 ? ? ? 14 ? ? ?0.146
>> 4 ? ? ? 15 ? ? ?0.174
>> 5 ? ? ? 14 ? ? ?0.158
>> 5 ? ? ? 15 ? ? ?0.141
>> 5 ? ? ? 16 ? ? ?0.178
>> 5 ? ? ?17 ? ?NA
>> 5 ? ? ? 18 ? ? ?0.172
>>
>> _______________________________________________
>> R-sig-mixed-models at r-project.org mailing list
>> https://stat.ethz.ch/mailman/listinfo/r-sig-mixed-models
>>
>
>
--
Robert Terwilliger
Physicist
Laboratory of Neurocognitive Development
Western Psychiatric Institute and Clinic
University of Pittsburgh Medical Center
Loeffler Building
121 Meyran Avenue #114
Pittsburgh, PA 15213
412.383.8174 - Office
412.383.8179 - Fax
em: raterwil at gmail.com
http://www.wpic.pitt.edu/research/lncd/