A conditional distribution describes how a variable is distributed after selecting cases with specified information. For Y|X = a, keep observations where X equals a, then examine their Y values. This differs from the unconditional distribution, which combines all observed cases. section 1.5.2
Selecting a subgroup
Suppose the selected cases have X = a and Y values 7.01 and 10.89. Their conditional distribution gives each observed value weight 1/2, so E[Y|X = a] = 8.95. Adding Z = B narrows the selection to Y = 10.89. section 1.5.2
Table 1.3 Simulated data conditioned on X = a
|
X
|
Y
|
Z
|
|---|---|---|
|
a
|
7.01
|
A
|
|
a
|
10.89
|
B
|
Inspect the rows remaining after conditioning the simulated data on X = a. section 1.5.2
For binary Y, the conditional expectation E[Y|X] equals the conditional probability P(Y|X). In general, E[Y|X = x] means calculating the average Y among cases with the chosen value x. section 1.5.2
