---
title: "(advanced) Local decorrelation for time series"
bibliography: "../inst/REFERENCES.bib"
csl: "../inst/apa-6th.csl"
output: 
  rmarkdown::html_vignette
description: >
  This vignette provides an overview of the local decorrelation technique 
  from time series with vanishing amount of correlations.
vignette: >
  %\VignetteIndexEntry{(advanced) Local decorrelation for time series}
  %\VignetteEngine{knitr::rmarkdown}
  \usepackage[utf8]{inputenc}
---


```{r, echo = FALSE, warning=FALSE, message = FALSE, results = 'hide'}
cat("this will be hidden; use for general initializations.\n")
library(superb)
options("superb.feedback" = c("warnings"))
library(ggplot2)
```

The decorrelation of the measures in repeated-measure designs is meant to have error bars
that are integrating the added power of using repeated-measures over independent groups.
In design with a few measurements, the correlation between the pairs of measurements is
indicative of the gain in statistical power. However, in time series, correlation is likely
to vanish as measurements get further spaced in time (the ``lag`` effect).

For example, consider a longitudinal study of adolescents over 10 years. The measurements that
are 6-month apart may show some correlations, but the two most separated measurements (say the first
at 8 years old and the second at 18 years old) are much less likely to preserve their correlations.

This vignette propose a solution. It is detailed in @cppf24.

## The structure of correlations

When repeated measures are obtained, one may compute the correlation matrix. The
correlation matrix is always composed of 1s along the main diagonal, as the correlation of a variable 
with itself is always 1. What is more interesting is what happen off the main diagonal.

In some situations, the correlations are fairly constant (stationary). When the variance are
further homogeneous, this correlation structure is known as ``compound symmetry``. Compound symmetry
is the simplest situation and also the easiest to analyze (with e.g., ANOVAs, alghough ANOVA really
requires ``sphericity``, a slightly different correlation structure).

In other situations, we might see that correlations near the main diagnonal are strong, but as we
distance from the diagonal (either in the upper-right or lower-left directions), the correlations slowly
vanishes, possibly reaching near-null values. This structure is known as an ``autoregressive covariance
structure of the first order`` or AR(1). In time series, that would indicate that the correlation of a 
measurement with the measurement just before or just after is high, but that the correlation between
a measurement and a distant measurement is weak.

## Implications for precision

Vanishing correlations means that comparing distant points in time will be performed with weaker statistical
precision and comparisons of close-by measures will benefit from much correlation (_correlation is your
friend_ when it comes to statistical inference).

In plotting curves, our objective may be to see how the points evolves, which imply that we are making multiple
comparisons of close-by points. If so, our visual tools should be based on the correlation (presumably high)
between these nearby points. If our objective is instead to compare far-distance points, the visual tools 
should incorporate the correlations of these distant points (presumably weak).

## How is correlation assessed then?

There are a few techniques to estimate the correlation in a correlation matrix. When it is assumed 
compound symmetric, the average of the pairwise correlations is satisfactory. When it is AR(1) however, 
the average won't do as the correlation is varying based on the lag.

We argue that a fit technique is to average the correlations using weights that are reducing with distance 
(excluding the main diagonal whose weight is set to 0). Any kernel (for example a gaussian kernel) can be
used to that end, as long as the width is kept smaller than the number of variables. We implemtented this
technique in ``superb``.


## Illustration with fMRI data

@w17 examined the finite impulse response obtained from an fMRI for two sites (frontal and 
parietal) and two event conditions (a cue-only condition and a cue+stimulus condition). The responses are
obtained over 19 time points (labeled 0 to 18) in these four conditions, resulting in 76 measurements.
There are 14 participants.

We first fetch the data from the main author's GitHub repository:

```{r}
fmri <- read.csv(url("https://raw.githubusercontent.com/mwaskom/seaborn-data/de49440879bea4d563ccefe671fd7584cba08983/fmri.csv"))
# sort the data...
fmri <- fmri[order(fmri$subject, fmri$event, fmri$region, fmri$timepoint),]
```

We are ready to make plots!

### A plot without decorrelation

The first plot is done without adjustments. By default, it shows the standalone 95% confidence interval.
The formula uses the long data format but specifies that the
observations are nested with subject numbers:

```{r, fig.height=4, fig.width=7, fig.cap = "**Figure 1**. Plot of the fMRI data with standalone confidence intervals."}
superb( 
    signal ~ timepoint + region + event | subject,
    fmri,
    plotStyle = "lineBand",
    pointParams = list(size=1,color="black"),
    lineParams = list(color="purple")
) + scale_x_discrete(name="Time", labels = 0:18) + 
scale_discrete_manual(aesthetic =c("fill","colour"), 
                      labels = c("frontal","parietal"), 
                      values = c("red","green")) +
theme_bw() + ylim(-0.15, +0.35) 
```

The `scale_x_discrete` is done to rename the ticks from 0 to 18 (they would start at 1 otherwise).
The `scale_discrete_manual` changes the color of the band (I hope you are color-blind, colors is not my
thing). The `plotStyle = "lineBand"` displays the confidence intervals as a band rather than as error bars.


### Plots with decorrelation

The decorrelation technique was first proposed by @lm94. Alternatives approaches were developped
in @c05 with @m08 [also see @c19]. They are known in ``superbPlot()`` as ``"LM"`` and
``"CM"`` respectively.

If you add this adjustment with this command, you get the following plot:

```{r, fig.height=4, fig.width=7, fig.cap = "**Figure 2**. Plot of the fMRI data with Cousineau-Morey decorrelation."}
superb( 
    signal ~ timepoint + region + event | subject,
    fmri,
    adjustments = list(decorrelation = "CM"), ## only new line
    plotStyle = "lineBand",
    pointParams = list(size=1,color="black"),
    lineParams = list(color="purple")
) + scale_x_discrete(name="Time", labels = 0:19) + 
scale_discrete_manual(aesthetic =c("fill","colour"), 
                      labels = c("frontal","parietal"), 
                      values = c("red","green"))+
theme_bw() + ylim(-0.15, +0.35) + 
showSignificance(c(6,7)+1, 0.3, -0.02, "n.s.?", panel=list(event=2)) 
```

As you may see, this plot and the previous one are nearly identical! This is because the average correlation
involving close-by and far-distant points is very weak (close to zero; replace CM with CA and a message
will return the average correlation in addition to a plot).

Because fMRI points are separated by time, close-by points ought to show some correlation. This is
where local decorrelation may be useful.

We repeat the above command, but this time ask for a local average of the correlation. We need to 
specify the radius of the kernel, which we do by adding an integer after the letters "LD". Here, 
we show the results with a narrow kernel, weighting far more adjacent points than points 3 time points
appart, obtained with ``"LD2"``:


```{r, fig.height=4, fig.width=7, fig.cap = "**Figure 3**. Plot of the fMRI data with local decorrelation."}
superb( 
    signal ~ timepoint + region + event | subject,
    fmri,
    adjustments = list(decorrelation = "LD2"),  ## CM replaced with LD2
    plotStyle = "lineBand",
    pointParams = list(size=1,color="black"),
    lineParams = list(color="purple")
) + scale_x_discrete(name="Time", labels = 0:19) + 
scale_discrete_manual(aesthetic =c("fill","colour"), 
                      labels = c("frontal","parietal"), 
                      values = c("red","green"))+
theme_bw() + ylim(-0.15, +0.35) + 
showSignificance(c(6,7)+1, 0.3, -0.02, "**!", panel=list(event=2)) 
```

As seen from the message, the correlations in nearby time points is about .50. It explains why the 
precision of the measures shrank so much (seen with confidence intervals that are much narrower). 
You can pick any two nearby points and run a paired t-test, the chances are high that you get
a significant result. 

As an example, consider the green curve, in condition cue+stimuli (i.e., bottom
panel), for time points 6 and 7. The confidence band suggest that these two points differ when
you examine the locally-decorrelated confidence intervals, but not when you examine the previous
two plots. Which is true? Let's run a t-test on paired sample.

```{r ThisChunk}
d1 <- dplyr::filter(fmri, event=="stim" & region=="parietal" & timepoint==6)
d2 <- dplyr::filter(fmri, event=="stim" & region=="parietal" & timepoint==7)

t.test(d1$signal, d2$signal, paired=TRUE)
```

## The radius parameter

You can vary the radius from 1 and above. The larger the radius, the smallest will be the benefit of correlation in the assessment of precision.
In the extreme, if you use a very large radius (e.g., "LD10000"), you will get the exact same
average correlation as with "CA" as now all the correlations are weighted almost identically.

Note that in the above computations, I reduced the number of messages displayed by ``superb()`` using
`options("superb.feedback" = "warnings" )`.

## Difference adjustments

In all three figures, we did not use the difference adjustment. Recall that this
adjustment is needed when the objective of the error bars (or error bands) is to
perform comparisons between pairs of conditions.

In the present example, the reader is very likely to perform comparisons between curves
so that the difference adjustment is very much needed. Simply add `purpose = "difference"`
in the `adjustments` list of the three examples above. You will see that of the three
plots above, only the locally-decorrelated one suggests significant differences between
the bottom curves on some time points, which is indeed what format tests indicate.

## In summary

Local decorrelation is a tool adapted to time series where nearby measurements are 
expected to show greater correlations than measurements separated by large amount 
of time. This is applicable among other to time series, longitudinal studies, 
fMRI studies (as the example above) and EEG studies (as the application described in @cppf24).


# References