## Exercise 1: Causal and statistical dependency.

For each of the following programs:

• Draw the dependency diagram (Bayes net). If you donâ€™t have software on your computer for doing this, Google Docs has a decent interface for creating drawings.

• Use informal evaluation order reasoning and the intervention method to determine causal dependency between A and B.

• Use conditioning to determine whether A and B are statistically dependent.

### a)

var a = flip();
var b = flip();
var c = flip(a && b ? .8 : .5);


### b)

var a = flip();
var b = flip(a ? .9 : .2);
var c = flip(b ? .7 : .1);


### c)

var a = flip();
var b = flip(a ? .9 : .2);
var c = flip(a ? .7 : .1);


### d)

var a = flip(.6);
var c = flip(.1);
var z = flip() ? a : c;
var b = z ? 'foo' : 'bar';


### e)

var examFairPrior = Bernoulli({p: .8});
var doesHomeworkPrior = Bernoulli({p: .8});
var examFair = mem(function(exam) { return sample(examFairPrior) });
var doesHomework = mem(function(student) { return sample(doesHomeworkPrior) });

var pass = function(student, exam) {
return flip(examFair(exam) ?
(doesHomework(student) ? .9 : .5) :
(doesHomework(student) ? .2 : .1));
}
var a = pass('alice', 'historyExam');
var b = pass('bob', 'historyExam');