# Examples

These examples demonstrate most of the functionality of the package, its typical usage, and how to make some plots you might want to use.

The examples:

• The Ising Model shows how to use the package to explore the autologistic probability distribution, without concern about covariates or parameter estimation.
• Clustered Binary Data (Small $n$) shows how to use the package for regression analysis when the graph is small enough to permit computation of the normalizing constant. In this case standard maximum likelihood methods of inference can be used.
• Spatial Binary Regression shows how to use the package for autologistic regression analysis for larger, spatially-referenced graphs. In this case pseudolikelihood is used for estimation, and a (possibly parallelized) parametric bootstrap is used for inference.

## The Ising Model

The term "Ising model" is usually used to refer to a Markov random field of dichotomous random variables on a regular lattice. The graph is such that each variable shares an edge only with its nearest neighbors in each dimension. It's a traditional model for magnetic spins, where the coding $(-1,1)$ is usually used. There's one parameter per vertex (a "local magnetic field") that increases or decreases the chance of getting a $+1$ state at that vertex; and there's a single pairwise parameter that controls the strength of interaction between neighbor states.

In our terminology it's just an autologistic model with the appropriate graph. Specifically, it's an ALsimple model: one with FullUnary type unary parameter, and SimplePairwise type pairwise parameter.

We can create such a model once we have the graph. For example, let's create the model on a 30-by-30 lattice:

using Autologistic, Random
Random.seed!(8888)
n = 30
G = makegrid4(n, n, (-1,1), (-1,1))
α = randn(n^2)
M1 = ALsimple(G.G, α)

Above, the line G = makegrid4(n, n, (-1,1), (-1,1)) produces an n-by-n graph with vertices positioned over the square extending from $-1$ to $1$ in both directions. It returns a tuple; G.G is the graph, and G.locs is an array of tuples giving the spatial coordinates of each vertex.

M1 = ALsimple(G.G, α) creates the model. The unary parameters α were intialized to Gaussian white noise. By default the pairwise parameter is set to zero, which implies independence of the variables.

Typing M1 at the REPL shows information about the model. It's an ALsimple type with one observation of length 900.

julia> M1Autologistic model of type ALsimple with parameter vector [α; λ].
Fields:
responses    900×1 Bool array
unary        900×1 FullUnary with fields:
α  900×1 array (unary parameter values)
pairwise     900×900×1 SimplePairwise with fields:
λ      [0.0] (association parameter)
G      the graph (900 vertices, 1740 edges)
count  1 (the number of observations)
A      900×900 SparseMatrixCSC (the adjacency matrix)
centering    none
coding       (-1, 1)
labels       ("low", "high")
coordinates  900-element vector of Tuple{Float64, Float64}

The conditionalprobabilities function returns the probablity of observing a $+1$ state at each vertex, conditional on the vertex's neighbor values. These can be visualized as an image, using a heatmap (from Plots.jl):

using Plots
condprobs = conditionalprobabilities(M1)
hm = heatmap(reshape(condprobs, n, n), c=:grays, aspect_ratio=1,
title="probability of +1 under independence")
plot(hm)

Since the association parameter is zero, there are no neighborhood effects. The above conditional probabilities are equal to the marginal probabilities.

Next, set the association parameters to 0.75, a fairly strong association level, to introduce a neighbor effect.

setpairwiseparameters!(M1, [0.75])

We can also generalize the Ising model by allowing the pairwise parameters to be different for each edge of the graph. The ALfull type represents such a model, which has a FullUnary type unary parameter, and a FullPairwise type pairwise parameter. For this example, let each edge's pairwise parameter be equal to the average distance of its two vertices from the origin.

using LinearAlgebra, Graphs
λ = [norm((G.locs[e.src] .+ G.locs[e.dst])./2) for e in edges(G.G)]
M2 = ALfull(G.G, α, λ)
Autologistic model of type ALfull with parameter vector [α; Λ].
Fields:
responses    900×1 Bool array
unary        900×1 FullUnary with fields:
α  900×1 array (unary parameter values)
pairwise     900×900×1 FullPairwise with fields:
λ      edge-ordered vector of association parameter values
G      the graph (900 vertices, 1740 edges)
count  1 (the number of observations)
Λ      900×900 SparseMatrixCSC (the association matrix)
centering    none
coding       (-1, 1)
labels       ("low", "high")
coordinates  900-element vector of Tuple{Float64, Float64}


A quick way to compare models with nonzero association is to observe random samples from the models. The sample function can be used to do this. For this example, use perfect sampling using a bounding chain algorithm.

s1 = sample(M1, method=perfect_bounding_chain)
s2 = sample(M2, method=perfect_bounding_chain)

Other options are available for sampling. The enumeration SamplingMethods lists them. The samples we have just drawn can also be visualized using heatmap:

pl1 = heatmap(reshape(s1, n, n), c=:grays, colorbar=false, title="ALsimple model");
pl2 = heatmap(reshape(s2, n, n), c=:grays, colorbar=false, title="ALfull model");
plot(pl1, pl2, size=(800,400), aspect_ratio=1)

In these plots, black indicates the low state, and white the high state. A lot of local clustering is occurring in the samples due to the neighbor effects. For the ALfull model, clustering is greater farther from the center of the square.

To see the long-run differences between the two models, we can look at the marginal probabilities. They can be estimated by drawing many samples and averaging them (note that running this code chunk can take a few minutes):

marg1 = sample(M1, 500, method=perfect_bounding_chain, verbose=true, average=true)
marg2 = sample(M2, 500, method=perfect_bounding_chain, verbose=true, average=true)
pl3 = heatmap(reshape(marg1, n, n), c=:grays,
colorbar=false, title="ALsimple model");
pl4 = heatmap(reshape(marg2, n, n), c=:grays,
colorbar=false, title="ALfull model");
plot(pl3, pl4, size=(800,400), aspect_ratio=1)
savefig("marginal-probs.png")

The figure marginal-probs.png looks like this:

The differences between the two marginal distributions are due to the different association structures, because the unary parts of the two models are the same. The ALfull model has stronger association near the edges of the square, and weaker association near the center. The ALsimple model has a moderate association level throughout.

As a final demonstration, perform Gibbs sampling for model M2, starting from a random state. Display a gif animation of the progress.

nframes = 150
gibbs_steps = sample(M2, nframes, method=Gibbs)
anim = @animate for i =  1:nframes
heatmap(reshape(gibbs_steps[:,i], n, n), c=:grays, colorbar=false,
aspect_ratio=1, title="Gibbs sampling: step $(i)") end gif(anim, "ising_gif.gif", fps=10) ## Clustered Binary Data (Small$n$) The retinitis pigmentosa data set (obtained from this source) is an opthalmology data set. The data comes from 444 patients that had both eyes examined. The data can be loaded with Autologistic.datasets: julia> using Autologistic, DataFrames, Graphsjulia> df = Autologistic.datasets("pigmentosa");julia> first(df, 6)6×9 DataFrame Row │ ID aut_dom aut_rec sex_link age sex psc eye va │ Int64 Int64 Int64 Int64 Int64 Int64 Int64 Int64 Int64 ─────┼────────────────────────────────────────────────────────────────────── 1 │ 7 0 0 0 27 0 1 0 0 2 │ 7 0 0 0 27 0 1 1 1 3 │ 8 0 1 0 44 1 0 0 0 4 │ 8 0 1 0 44 1 0 1 0 5 │ 19 0 0 0 24 0 1 0 1 6 │ 19 0 0 0 24 0 1 1 1julia> describe(df)9×7 DataFrame Row │ variable mean min median max nmissing eltype │ Symbol Float64 Int64 Float64 Int64 Int64 DataType ─────┼─────────────────────────────────────────────────────────────────── 1 │ ID 1569.22 7 1466.5 3392 0 Int64 2 │ aut_dom 0.0990991 0 0.0 1 0 Int64 3 │ aut_rec 0.141892 0 0.0 1 0 Int64 4 │ sex_link 0.0608108 0 0.0 1 0 Int64 5 │ age 34.2342 6 32.5 80 0 Int64 6 │ sex 0.576577 0 1.0 1 0 Int64 7 │ psc 0.432432 0 0.0 1 0 Int64 8 │ eye 0.5 0 0.5 1 0 Int64 9 │ va 0.504505 0 1.0 1 0 Int64 The response for each eye is va, an indicator of poor visual acuity (coded 0 = no, 1 = yes in the data set). Seven covariates were also recorded for each eye: • aut_dom: autosomal dominant (0=no, 1=yes) • aut_rec: autosomal recessive (0=no, 1=yes) • sex_link: sex-linked (0=no, 1=yes) • age: age (years, range 6-80) • sex: gender (0=female, 1=male) • psc: posterior subscapsular cataract (0=no, 1=yes) • eye: which eye is it? (0=left, 1=right) The last four factors are relevant clinical observations, and the first three are genetic factors. The data set also includes an ID column with an ID number specific to each patient. Eyes with the same ID come from the same person. The natural unit of analysis is the eye, but pairs of observations from the same patient are "clustered" because the occurrence of acuity loss in the left and right eye is probably correlated. We can model each person's two va outcomes as two dichotomous random variables with a 2-vertex, 1-edge graph. julia> G = Graph(2,1){2, 1} undirected simple Int64 graph Each of the 444 bivariate observations has this graph, and each has its own set of covariates. If we include all seven predictors, plus intercept, in our model, we have 2 variables per observation, 8 predictors, and 444 observations. Before creating the model we need to re-structure the covariates. The data in df has one row per eye, with the variable ID indicating which eyes belong to the same patient. We need to rearrange the responses (Y) and the predictors (X) into arrays suitable for our autologistic models, namely: • Y is$2 \times 444$with one observation per column. • X is$2 \times 8 \times 444$with one$2 \times 8$matrix of predictors for each observation. The first column of each predictor matrix is an intercept column, and columns 2 through 8 are for aut_dom, aut_rec, sex_link, age, sex, psc, and eye, respectively. X = Array{Float64,3}(undef, 2, 8, 444); Y = Array{Float64,2}(undef, 2, 444); for i in 1:2:888 patient = Int((i+1)/2) X[1,:,patient] = [1 permutedims(Vector(df[i,2:8]))] X[2,:,patient] = [1 permutedims(Vector(df[i+1,2:8]))] Y[:,patient] = convert(Array, df[i:i+1, 9]) end For example, patient 100 had responses julia> Y[:,100]2-element Vector{Float64}: 1.0 0.0 Indicating visual acuity loss in the left eye, but not in the right. The predictors for this individual are julia> X[:,:,100]2×8 Matrix{Float64}: 1.0 0.0 0.0 0.0 18.0 1.0 1.0 0.0 1.0 0.0 0.0 0.0 18.0 1.0 1.0 1.0 Now we can create our autologistic regression model. model = ALRsimple(G, X, Y=Y) Autologistic regression model of type ALRsimple with parameter vector [β; λ]. Fields: responses 2×444 Bool array unary 2×444 LinPredUnary with fields: X 2×8×444 array (covariates) β 8-element vector (regression coefficients) pairwise 2×2×444 SimplePairwise with fields: λ [0.0] (association parameter) G the graph (2 vertices, 1 edges) count 444 (the number of observations) A 2×2 SparseMatrixCSC (the adjacency matrix) centering none coding (-1, 1) labels ("low", "high") coordinates 2-element vector of Tuple{Float64, Float64}  This creates a model with the "simple pairwise" structure, using a single association parameter. The default is to use no centering adjustment, and to use coding$(-1,1)\$ for the responses. This "symmetric" version of the model is recommended for a variety of reasons. Using different coding or centering choices is only recommended if you have a thorough understanding of what you are doing; but if you wish to use different choices, this can easily be done using keyword arguments. For example, ALRsimple(G, X, Y=Y, coding=(0,1), centering=expectation) creates the "centered autologistic model" that has appeared in the literature (e.g., here and here).

The model has nine parameters (eight regression coefficients plus the association parameter). All parameters are initialized to zero:

julia> getparameters(model)9-element Vector{Float64}:
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0

When we call getparameters, the vector returned always has the unary parameters first, with the pairwise parameter(s) appended at the end.

Because there are only two vertices in the graph, we can use the full likelihood (fit_ml! function) to do parameter estimation. This function returns a structure with the estimates as well as standard errors, p-values, and 95% confidence intervals for the parameter estimates.

out = fit_ml!(model)
Autologistic model fitting results. Its non-empty fields are:
estimate       9-element vector of parameter estimates
se             9-element vector of standard errors
pvalues        9-element vector of 2-sided p-values
CIs            9-element vector of 95% confidence intervals (as tuples)
optim          the output of the call to optimize()
Hinv           the inverse of the Hessian, evaluated at the optimum
kwargs         extra keyword arguments passed to sample()
convergence    true
Use summary(fit; [parnames, sigdigits]) to see a table of estimates.
For pseudolikelihood, use oneboot() and addboot!() to add bootstrap after the fact.

To view the estimation results, use summary:

summary(out, parnames = ["icept", "aut_dom", "aut_rec", "sex_link", "age", "sex",
"psc", "eye", "λ"])
name       est        se        p-value   95% CI
icept      -0.264     0.0972    0.00661    (-0.454, -0.0735)
aut_dom    -0.167     0.0932    0.0734     (-0.349, 0.0158)
aut_rec     0.104     0.0787    0.188     (-0.0507, 0.258)
sex_link    0.301     0.126     0.0168     (0.0542, 0.547)
age         0.00492   0.00184   0.0076    (0.00131, 0.00853)
sex         0.0207    0.0557    0.711     (-0.0885, 0.13)
psc         0.139     0.0588    0.0185     (0.0233, 0.254)
eye         0.0273    0.119     0.819      (-0.207, 0.261)
λ           0.818     0.0655    0.0          (0.69, 0.947)

From this we see that the association parameter is fairly large (0.818), supporting the idea that the left and right eyes are associated. It is also highly statistically significant. Among the covariates, sex_link, age, and psc are all statistically significant.

## Spatial Binary Regression

ALR models are natural candidates for analysis of spatial binary data, where locations in the same neighborhood are more likely to have the same outcome than sites that are far apart. The hydrocotyle data provide a typical example. The response in this data set is the presence/absence of a certain plant species in a grid of 2995 regions covering Germany. The data set is included in Autologistic.jl:

julia> using Autologistic, DataFrames, Graphsjulia> df = Autologistic.datasets("hydrocotyle")2995×5 DataFrame
Row │ X      Y      altitude  temperature  obs
│ Int64  Int64  Float64   Float64      Int64
──────┼────────────────────────────────────────────
1 │    16      1   1.71139     8.2665        1
2 │    15      2   1.8602      8.15482       1
3 │    16      2   1.7858      8.09898       1
4 │    17      2   1.7858      8.09898       0
5 │    18      2   1.7858      8.09898       0
6 │    19      2   1.8602      8.09898       1
7 │    15      3   1.8602      8.15482       1
8 │    16      3   1.7858      8.15482       1
⋮   │   ⋮      ⋮       ⋮           ⋮         ⋮
2989 │    26     78  16.4442      2.6269        0
2990 │    27     78  15.3281      3.5203        0
2991 │    28     78  17.6347      1.90102       0
2992 │    31     78  16.3698     -0.5           0
2993 │    32     78  15.4025     -0.5           0
2994 │    33     78  16.3698      0.281726      0
2995 │    27     79  18.23        1.95685       0
2980 rows omitted

In the data frame, the variables X and Y give the spatial coordinates of each region (in dimensionless integer units), obs gives the presence/absence data (1 = presence), and altitude and temperature are covariates.

We will use an ALRsimple model for these data. The graph can be formed using makespatialgraph:

locations = [(df.X[i], df.Y[i]) for i in 1:size(df,1)]
g = makespatialgraph(locations, 1.0)

makespatialgraph creates the graph by adding edges between any vertices with Euclidean distance smaller than a cutoff distance (Graphs.jl has a euclidean_graph function that does the same thing). For these data arranged on a grid, a threshold of 1.0 will make a 4-nearest-neighbors lattice. Letting the threshold be sqrt(2) would make an 8-nearest-neighbors lattice.

We can visualize the graph, the responses, and the predictors using GraphRecipes.jl (there are several other options for plotting graphs as well).

using GraphRecipes, Plots

# Function to convert a value to a gray shade
makegray(x, lo, hi) = RGB([(x-lo)/(hi-lo) for i=1:3]...)

# Function to plot the graph with node shading determined by v.
# Plot each node as a square and don't show the edges.
function myplot(v, lohi=nothing)
colors = lohi==nothing ? makegray.(v, minimum(v), maximum(v)) : makegray.(v, lohi[1], lohi[2])
return graphplot(g.G, x=df.X, y=df.Y, background_color = :lightblue,
marker = :square, markersize=2, markerstrokewidth=0,
markercolor = colors, yflip = true, linecolor=nothing)
end

# Make the plot
plot(myplot(df.obs), myplot(df.altitude), myplot(df.temperature),
layout=(1,3), size=(800,300), titlefontsize=8,
title=hcat("Species Presence (white = yes)", "Altitude (lighter = higher)",
"Temperature (lighter = higher)"))