# 1 Intro

When examining the relationship between a composition and an outconme, we are often are interested in how an outcome changes when a fixed unit in the composition (e.g., minutes of behaviours during a day) is reallocated from one component to another. The Compositional Isotemporal Substitution Analysis can be used to estimate this change. The `multilevelcoda` package implements this method in a multilevel framework and offers functions for both between- and within-person levels of variability. We discuss 4 different substitution models in this vignette.

We will begin by loading necessary packages, `multilevelcoda`, `brms` (for models fitting), doFuture (for parallelisation), and data sets `mcompd` (simulated compositional sleep and wake variables), `sbp` (sequential binary partition), and `psub` (base possible substitution).

``````library(multilevelcoda)
library(brms)
library(doFuture)

data("mcompd")
data("sbp")
data("psub")

options(digits = 3) # reduce number of digits shown``````

# 2 Fitting main model

Let’s fit our main `brms` model predicting `Stress` from both between and within-person sleep-wake behaviours (represented by isometric log ratio coordinates), with sex as a covariate, using the `brmcoda()` function. We can compute ILR coordinate predictors using `complr()` function.

``````cilr <- complr(data = mcompd, sbp = sbp,
parts = c("TST", "WAKE", "MVPA", "LPA", "SB"), idvar = "ID", total = 1440)

m <- brmcoda(complr = cilr,
formula = Stress ~ bilr1 + bilr2 + bilr3 + bilr4 +
wilr1 + wilr2 + wilr3 + wilr4 + Female + (1 | ID),
cores = 8, seed = 123, backend = "cmdstanr")``````

A `summary()` of the model results.

``````summary(m)
#>  Family: gaussian
#>   Links: mu = identity; sigma = identity
#> Formula: Stress ~ bilr1 + bilr2 + bilr3 + bilr4 + wilr1 + wilr2 + wilr3 + wilr4 + Female + (1 | ID)
#>    Data: tmp (Number of observations: 3540)
#>   Draws: 4 chains, each with iter = 2000; warmup = 1000; thin = 1;
#>          total post-warmup draws = 4000
#>
#> Multilevel Hyperparameters:
#> ~ID (Number of levels: 266)
#>               Estimate Est.Error l-95% CI u-95% CI Rhat Bulk_ESS Tail_ESS
#> sd(Intercept)     0.99      0.06     0.87     1.11 1.00     1574     2367
#>
#> Regression Coefficients:
#>           Estimate Est.Error l-95% CI u-95% CI Rhat Bulk_ESS Tail_ESS
#> Intercept     2.65      0.48     1.69     3.56 1.00     1494     2202
#> bilr1         0.11      0.32    -0.53     0.74 1.00      967     1699
#> bilr2         0.50      0.34    -0.16     1.15 1.00     1081     2071
#> bilr3         0.11      0.21    -0.31     0.53 1.00     1059     2057
#> bilr4         0.04      0.28    -0.51     0.60 1.00     1266     2174
#> wilr1        -0.34      0.12    -0.58    -0.09 1.00     2797     2882
#> wilr2         0.05      0.13    -0.21     0.32 1.00     3134     2299
#> wilr3        -0.10      0.08    -0.26     0.06 1.00     2712     2653
#> wilr4         0.24      0.10     0.04     0.44 1.00     2963     2874
#> Female       -0.41      0.17    -0.77    -0.07 1.00     1364     1823
#>
#> Further Distributional Parameters:
#>       Estimate Est.Error l-95% CI u-95% CI Rhat Bulk_ESS Tail_ESS
#> sigma     2.38      0.03     2.33     2.44 1.00     4866     2981
#>
#> Draws were sampled using sample(hmc). For each parameter, Bulk_ESS
#> and Tail_ESS are effective sample size measures, and Rhat is the potential
#> scale reduction factor on split chains (at convergence, Rhat = 1).``````

We can see that the first and forth within-person ILR coordinates were both associated with stress. Interpretation for multilevel ILR coordinates can often be less intuitive. For example, the significant coefficient for wilr1 shows that the within-person change in sleep behaviours (sleep duration and time awake in bed combined), relative to wake behaviours (moderate to vigorous physical activity, light physical activity, and sedentary behaviour) on a given day, is associated with stress. However, as there are several behaviours involved in this coordinate, we don’t know the within-person change in which of them drives the association. It could be the change in sleep, such that people sleep more than their own average on a given day, but it could also be the change in time awake. Further, we don’t know about the specific changes in time spent across behaviours. That is, if people sleep more, what behaviour do they spend less time in?

This is common issue when working with multilevel compositional data as ILR coordinates often contains information about multiple compositional components. To gain further insights into these associations and help with interpretation, we can conduct post-hoc analyses using the substitution models from our `multilevel` package.

# 3 Substitution models

`multilevelcoda` package provides `2` different methods to compute substitution models, via the `substitution()` function.

Basic substitution models:

• Between-person substitution
• Within-person substitution

Average marginal substitution models:

• Average marginal between-person substitution
• Average marginal within-person substitution

Tips: Substitution models are often computationally demanding tasks. You can speed up the models using parallel execution, for example, using `doFuture` package.

## 3.1 Basic Substitution Analysis

The below example examines the changes in stress for different pairwise substitution of sleep-wake behaviours for a period of 1 to 5 minutes, at between-person level. We specify `level = between` to indicate substitutional change would be at the between-person level, and `ref = "grandmean"` to indicate substitution model using the grand compositional mean as reference composition. If your model contains covariates, `substitution()` will average predictions across levels of covariates as the default.

``````subm1 <- substitution(object = m, delta = 1:10,
ref = "grandmean", level = c("between", "within"))``````

Output fr