[R] Re: comparing predicted sequence A'(t) to observed sequence A(t)

[Christian Jost]

>From: Suresh Krishna <ssk2031 at columbia.edu>
>Subject: [R] comparing predicted sequence A'(t) to observed sequence
>	A(t)
>To: r-help at stat.math.ethz.ch
>Message-ID: <420DE463.8080009 at columbia.edu>
>Content-Type: text/plain; charset=ISO-8859-1; format=flowed
>
>
>Hi,
>
>I have a question that I have not been succesful in finding a definitive
>answer to; and I was hoping someone here could give me some pointers to
>the right place in the literature.
>
>A. We have 4 sets of data, A(t), B(t), C(t), and D(t). Each of these
>consists of a series of counts obtained in sequential time-intervals: so
>   for example, A(t) would be something like:
>
>Count A(t):  25,    28,    26,   34   ......
>Time (ms):  0-10, 10-20, 20-30, 30-40 .......
>
>Each count in the series A(t) is obtained by summing the total number of
>observed counts over multiple (say 50), independent repetitions of that
>time-series. These counts are generally known to be Poisson distributed,
>and the 4 processes A(t), B(t), C(t) and D(t) are independent of each other.
>
>B. It appears on visual observation that the following relationship
>holds; and such a relationship would also be expected on mechanistic
>considerations.
>
>A(t) = B(t) + C(t) - D(t)
>
>We now want to test this hypothesis statistically.
>
>Because successive counts in the sequence are likely to be correlated,
>isnt it true that none of these methods are valid ? Perhaps for other
>reasons as well ?
>
>a)Doing a chi-squared test to see if the predicted curve for A(t)
>deviates significantly from the observed A(t); this also seems to not
>take the variability of the predicted curve into account.
>
>b)Doing a regression of the predicted values of A(t) against the actual
>values of A(t) and checking for deviations of slope from 1 and intercept
>from 0 ? Here, in addition to lack of independence, the fact that
>X-values are not fixed (i.e. are variable) and the fact that X and Y are
>Poisson distributed counts should also be taken into account, right ?
>
>I would be very grateful if someone could point me to methods to handle
>this kind of situation, or where to look for them. Is there something in
>the time-series literature, for instance ?
>
>

This is a frequent problem I also encounter when wanting to compare 
two dynamic processes (e.g. temporal evolution of number of ants on 
two branches). To my knowledge there is no general statistical way to 
compare these two time series. But in your case you might try a 
repeated measure anova, e.g. to compare A(t) against B(t)+C(t)-D(t), 
put in a first column 'counts' the counts for A and then for B+C-D, 
in a second column 'time' the correspoding t, in a third column 
'series' mark the A measures by "A" and the B+C-D measures by "BCD", 
then run an anova
summary(aov(counts ~ series:time + Error(series)))

This works if there are replicates of conditions "A" and "BDC", but I 
am not a statistitian and am not sure whether it applies to your case 
(though, you seem to have repetitions, so you might use this 
information instead of only looking at the sums).
For a hands-on example with behavioural data of mice (with or without 
treatment, 4 training session for each mouse, does treatment affect 
training) see
http://cognition.ups-tlse.fr/_christian/M7P14M/TP7/TP-Anova.pdf
with the data in
http://cognition.ups-tlse.fr/_christian/M7P14M/TP7/tp-anova.rda
(well, its in french, but the R formulas should be understandable ;-)

Well, as I said, I am not a statistitian, there might be a logical 
flaw in applying repeated measures anova to time series, if anybody 
out there sees one please tell us ;-)

Best, Christian.
-- 
***********************************************************
http://cognition.ups-tlse.fr/vas-y.php?id=chj  jost at cict.fr
Christian Jost                                   (PhD, MdC)
Centre de Recherches sur la Cognition Animale
Universite Paul Sabatier, Bat IV R3
118 route de Narbonne
31062 Toulouse cedex 4, France
Tel: +33 5 61 55 64 37   Fax: +33 5 61 55 61 54