STEP 1: write down the causal model as a directed graph

X1

X2

X3

X4

X5

4

5

a13

a23

a34

a35

b3

3

b4

b5

s45|3

OPTIONAL STEP: center each variable about its mean (Xi-Xi)

X1

X2

X3

X4

X5

4

5

a13

a23

a34

a35

b3

3

b4

b5

s45|3

Exogenous variables: variables that have no explicit causes in the model

Endogenous variables: variables that are caused by other variables in the model

X1

X2

X3

X4

X5

4

5

a13

a23

a34

a35

b3

3

b4

b5

s45|3

STEP 2: translate this to a series of structured equations with free parameters

X1=N(0,)

X2=N(0,)

X3=a13X1+a23X2+b33

3=N(0,1)

4=N(0,1)

5=N(0,1)

X4=a34X3+b44 X5=a35X3+b55

Cov(X1,X2)=Cov(X1,3)=Cov(X1, 4)=Cov(X1, 5)= Cov(X2,3)=Cov(X2, 4)=Cov(X2, 5)=Cov(3,4)=Cov(3, 5)=0Cov(4, 5)=45

freeparameter

STEP 3: Derive the predicted variance and covariance between each pair of variables in the model, respecting the constraints implied by the causal graph, using covariance algebra.

=

X1 X2 X3 X4 X5

X1 S1 0 a13S1 a13a34S1 a23a35S1X2 S2 a23S2 a13a34S2 a23a35S2X3 S3 a34S3 a35S3X4 S4 a23S3+

b4b5Cov(4,5)X5 S5

STEP 4: Estimate the free parameters by minimizing the difference between the observed and predicted variances and covariances.

Observedvariances &covariances(S)

Predictedvariances &covariances((free))

Maximumlikelihoodestimation

Estimates of the free parameters (path coefficients,error variances, freecovariances) that make the predicted covariance matrix() as close as possibleto the observed variances and covariances, while respecting the constraintsrequired by d-separation (I.e. the causal structure).

STEP 6: Look at the remaining differences between the observed and predicted covariance matrices, and calculate the probability of having observed this difference, assuming that these differences should be the same except for random sampling variation.

X2ML (maximum likelihood chi-square statistic)

degrees of freedom = V(V+1)/2 - #free parameters

STEP 7: If the calculated probability is less than the chosen significance level (eg 0.05) then the data did not come from this causal process, and the model must be rejected; otherwise the data support the model.

Overall association between X and Y

Effects of causal ancestors on causal

descendents

Direct effects Indirect effects

Effects due to a common causal ancestor

Unresolved causal

relationships

A

B

C

D