talking points

Talking PointsJoseph Ramsey

LiNGAMMost of the algorithms included in Tetrad (other than

KPC) assume causal graphs are to be inferred from conditional independence tests. Usually tests that assume linearity and Gaussianity.

LiNGAM uses a different approach. Assumes linearity and non-Gaussianity.

Runs Independent Components Analysis (ICA) to estimate the coefficient matrix.

Rearranges the coefficient matrix to get a causal order.Prunes weak coefficients by setting them to zero.

ICA Although complicated, the basic idea is very simple.

a11 X1 + ... + a1n Xn = e1 ... an1 X1 + ... + ann Xn = en

Assume e1,...,en are i.i.d. Try to maximize the non-Gaussianity of

w1 X1 + ... + wn Xn = ? There are n ways to do it up to symmetry! (Cf. Central

Limit Theorem, Hyavarinen et al., 2002) You can use the coefficients for e1, or for e2, or for... All other linear combinations of e1,...,en are more Gaussian.

ICAThis equation is usually denoted Wx = s

But also X = BX + s where B is the coefficient matrix So Wx = (I – B)x = e s is the vector of independent components x is the vector of variables

Just showed that under strong conditions we can estimate W. So we can estimate B! (But with unknown row order) Using assumptions of linearity and non-Gaussianity (of all

but one variable) alone.More sophisticated analyses allow errors to be non-i.i.d.

LiNGAMLiNGAM runs ICA to estimate the coefficient matrix B.The order of the errors is not fixed by ICA, so some

rearranging of the B matrix needs to be done.Rows of the B matrix are swapped so the it is lower

triangular. a[i][j] should be non-zero (representing an edge) just in

case ij Typically, a cutoff is used to determine if a matrix

element is zero. The rearranged matrix corresponds to the idea of a

causal order.

LiNGAMOnce you know which nodes are adjacent in the

graph and what the causal order is, you can infer a complete DAG.

Review:Use data from a linear non-Gaussian model (all but

one variable non-Gaussian) Infer a complete DAG (more than a pattern!)

Generalized SEMIn order to try LiNGAM we first need to simulate

some linear non-Gaussian data, for which we will need to use the Generalized SEM Model.

The Generalized SEM is a generalization of the linear SEM model.Allows for arbitrary connection functionsAllows for arbitrary distributionsSimulation from cyclic models supported.

Hand OnCreate a DAG.Parameterize it as a Generalized SEM.Open the Generalized SEM and select Apply

Templates from the Tools menu.Apply the default template to variables, which will

make them all linear functions.For errors, select a non-Gaussian distribution, such

as U(0, 1).Save.

Hand OnAttach a Generalized SEM IM.Attach a data set, simulate 1000 points.Attach a Search box and run LiNGAM.Attach another search box to Data and run PC.Compare PC to LiNGAM.

Special Variants of Algorithms

PC PatternPC Pattern enforces the requirement that the

output of the algorithm will be a pattern.PCD

PCD adds corrective code to PC for the case where some variables stand in deterministic relationships.

This results in fewer edges being removed from the graph.For example, if X _||_ Y | Z but Z determines Y, X---Y is

not taken out.

Special Variants of Algorithms

CPCThe PC algorithm may jump too quickly to the

conclusion that a collider and noncolliders should be oriented, X->Y<-Z, X---Y---Z

The CPC algorithm uses a much more conservative test for colliders and noncolliders, double and triple checking to make sure they should be oriented, against different adjacents to X and to Z.

The result is a graph with fewer but more accurate orientations.

Hands OnSimulate data from a “complicated” DAG using a SEM

IM. Choose the Search from Simulated Data item from the

Templates menu. Make a random 20 node 20 edge DAG. Parameterize as a linear SEM, accepting defaults. Run CPC. Attach another search box to data. Run PC. Layout the PC graph using Fruchterman-Reingold. Copy the layout to the CPC graph. Open PC and CPC simultaneously and note the differences.

Special Variants of Algorithms

CFCISame idea as for CPC but for FCI instead.

KPCThe PC algorithm typically uses independence

tests that assume linearity.The KPC algorithm makes two changes:

It uses a non-parametric independence test.It adds some steps to orient edges that are

unoriented in the PC pattern.

Special Variants of Algorithms

PcLiNGAM If some variables are Gaussian (more than one),

others non-Gaussian, this algorithm applies.Runs PC, then orients the unoriented edges (if

possible) using non-Gaussianity.LiNG

Extends LiNGAM to orient cycles using non-Gaussianity

Special Variants of Algorithms

JCPCUses a Markov blanket style test to add/remove

individual edges, using CPC style orientation.Allows individual adjacencies in the graph to be

revised from the initial estimate using the PC adjacency search.

Time Series Simulation (Hands On)

Tetrad includes support for doing time series simulations.

First, one creates a time series graph.

Then one parameterizes the time series graph as a SEM.

Then one instantiates the SEM.Then one simulates data from the SEM

Instantiated Model.

Time Series SimulationOne can, e.g., calculate a vector auto-regression for it.

(One can do this as well from time series data loaded in.) Attach a data manipulation box to the data. Select vector auto-regression. Attach a search and run GES. Should give the graph among concurrent variables.

One can create staggered time series data and run GES. Attach a data manipulation box. Select create time series data. Attach a search box and run GES. Should give the time lag graph with some extra edges in the

highest lag.

Command Line TetradWe don’t have an extensive command line

interface programmed, but what we do have has proven useful to many people.

We have a command line interface for a number of the basic search algorithms in Tetrad.

We also have a command line interface for the IMaGES algorithm.

Some upcoming version of Tetrad will include a more extensive command line interface.

How to get itGo to the Tetrad downloads directory,

http://www.phil.cmu.edu/projects/tetrad_download/download/

Look for files beginning with the prefix “tetradcmd-”.

Pick the one with the latest version.

How to run a search at the command line...

Example: java -jar tetradcmd-4.3.3-1.jar -data munin1.txt -

datatype discrete –algorithm fci -depth 3 -significance 0.0

Command line options-data: Gives the data file-datatype: continuous or discrete (mixed not

supported)-algorithm: pc, cpc, fci, cfci, ccd, ges-depth: Default is -1 (unlimited)-significance: Default is 0.05... Some others.

IMaGES command lineIMaGES (which I’ll talk about) is a more

specialized algorithm and uses its own command line interface.

Email me if you’d like to use it.

Tetrad SourceWe regularly get requests for the Tetrad source

code.The secret is, it’s online, freely available, you

just have to know where to look!Again, look in the Tetrad downloads directoryLook for the latest “dist” (distribution) file, unzip

it.

SourceAll of the code will be in the distribution, except

for private project code.This can be useful if you want to modify or

extend algorithms, or if you want to set up specific kinds of testing, or if the command line tools provided are insufficient for your needs.

JavaThe source code is in Java, which can be

interfaced with several other platforms with a bit of work.Matlab, R, Mathematica, also can be called from

the command line programmatically from various languages.

Also, since it’s in Java, it’s cross-platform compatible, so it will probably run on your machine.