# Chapter 3 Multi-Way Contingency Tables

Multi-way contingency tables are very common in practice, derived by the presence of more than two cross-classification variables.

## 3.1 Description

### 3.1.1 Three-way Tables

• Consider an $$I \times J \times K$$ contingency table $$(n_{ijk})$$ for $$i = 1,...,I$$, $$j = 1,...,J$$ and $$k = 1,...,K$$, with classification variables $$X$$ (the rows), $$Y$$ (the columns) and $$Z$$ (the layers) respectively.

• A schematic of a generic $$X \times Y \times Z$$ contingency table of counts is shown in Figure 3.1.

• We can define the joint probability distribution of $$(X,Y,Z)$$ as $$$\pi_{ijk} = P(X=i, Y=j, Z=k)$$$

• Proportions, observed and random counts are defined similarly to the $$I \times J$$ contingency table cases….

#### 3.1.1.1 Example

The table in Figure 3.2 shows an example of a 3-way contingency table. This hypothetical data cross-classifies the response ($$Y$$) to a treatment drug ($$X$$) at one of two different clinics ($$Z$$).

#### 3.1.1.2 Partial/Conditional Tables

• Partial, or conditional, tables involve fixing the category of one of the variables.

• We denote the fixed variable in parentheses.

• For example, the set of $$XY$$-partial tables consist of the $$K$$ corresponding two-way layers, denoted as $$(n_{ij(k)})$$ for $$k = 1,...,K$$.

• $$XZ$$ and $$YZ$$- partial tables are denoted as $$(n_{i(j)k})$$ and $$(n_{(i)jk})$$ respectively.

• Partial/conditional probabilities: $$$\pi_{ij(k)} = \pi_{ij|k} = P(X=i, Y=j | Z=k) = \frac{\pi_{ijk}}{\pi_{++k}} \qquad k = 1,...,K$$$

• Partial/conditional proportions: $$$p_{ij(k)} = p_{ij|k} = \frac{n_{ijk}}{n_{++k}} \qquad k = 1,...,K$$$

#### 3.1.1.3 Marginal Tables

• Marginal tables involve summing over all possible categories of a particular variable.

• We denote such summation using a $$+$$ (as before).

• For example, the $$XY$$- marginal table is $$(n_{ij+}) = (\sum_k n_{ijk})$$.

• $$XZ$$ and $$YZ$$- marginal tables are denoted as $$(n_{i+k})$$ and $$(n_{+jk})$$ respectively.

• Marginal probabilities: $$$\pi_{ij} = \pi_{ij+} = P(X=i, Y=j) = \sum_{k=1}^K \pi_{ijk}$$$

• Marginal proportions: $$$p_{ij} = p_{ij+} = \sum_{k=1}^K p_{ijk}$$$

#### 3.1.1.4 Marginal Vectors

• Information on the single classification variables is summarised in the marginal vectors $$(n_{1++},...,n_{I++})$$, $$(n_{+1+},...,n_{+J+})$$ and $$(n_{++1},...,n_{++K})$$ respectively.

### 3.1.2 Generic Multiway Tables

• A multiway $$I_1 \times I_2 \times ... \times I_q$$ contingency table will analogously be denoted as $$(n_{i_1i_2...i_q})$$, $$i_l = 1,...,I_l$$, $$l = 1,...,q$$.

• The definition of partial and marginal tables also follow analogously.

• For example, $$(n_{i_1+(i_3)i_4(i_5)})$$ denotes the two-way partial marginal table obtained by summing over all levels/categories of $$i_2$$ for a fixed level/category of $$i_3$$ and $$i_5$$.

## 3.2 Odds Ratios

• Conditional and marginal odds ratios can be defined for any two-way conditional or marginal probabilities table of a multi-way $$I_1 \times I_2 \times ... \times I_q$$ table with $$I_l \geq 2$$, $$l = 1,...,q$$.

• In this case, the conditional and marginal odds ratios are defined as odds ratios for two-way tables of size $$I \times J$$.

• Thus, as defined for general two-way tables in Sections 2.5 and 2.6.2, there will be a (not unique) minimal set of odds ratios of nominal, local, cumulative, or global type.

• For example, for an $$I \times J \times K$$ table, the $$XY$$ local odds ratios conditional on $$Z$$ are defined by $$$r_{ij(k)}^{XY} = \frac{\pi_{ijk}\pi_{i+1,j+1,k}}{\pi_{i+1,j,k}\pi_{i,j+1,k}} \qquad i = 1,...,I-1 \quad j = 1,...,J-1 \quad k = 1,...,K$$$ and the $$XY$$-marginal local odds ratios are defined by $$$r_{ij}^{XY} = \frac{\pi_{ij+}\pi_{i+1,j+1,+}}{\pi_{i+1,j,+}\pi_{i,j+1,+}} \qquad i = 1,...,I-1 \quad j = 1,...,J-1$$$

• The conditional and marginal odds ratios of other types, like nominal, cumulative and global, are defined analogously.

## 3.3 Types of Independence

• Let $$(n_{ijk})$$ be an $$I \times J \times K$$ contingency table of observed frequencies with row, column and layer classification variables $$X$$, $$Y$$ and $$Z$$ respectively.

• We consider various types of independence that could exist among these three variables.

### 3.3.1 Mutual Independence

• $$X$$, $$Y$$ and $$Z$$ are mutually independent if and only if $$$\pi_{ijk} = \pi_{i++} \pi_{+j+} \pi_{++k} \qquad i = 1,...,I \quad j = 1,...,J \quad k = 1,...,K \tag{3.1}$$$

• Such mutual independence can be symbolised as $$[X,Y,Z]$$.

#### 3.3.1.1 Example

• Following the example of Section 3.1.1.1, mutual independence would mean that clinic, drug and response were independent of each other.

• In other words, knowledge of the values of one variable doesn’t affect the probabilities of the levels of the others.

### 3.3.2 Joint Independence

• If $$Y$$ is jointly independent from $$X$$ and $$Z$$ (without these two being necessarily independent), then $$$\pi_{ijk} = \pi_{+j+} \pi_{i+k} \qquad i = 1,...,I \quad j = 1,...,J \quad k = 1,...,K \tag{3.2}$$$

• Such joint independence can be symbolised as $$[Y,XZ]$$.

• By symmetry, there are two more hypotheses of this type, which can be expressed in a symmetric way to Equation (3.2) for $$X$$ or $$Z$$ being jointly independent from the remaining two variables. These could be symbolised as $$[X, YZ]$$ and $$[Z, XY]$$ respectively.

#### 3.3.2.1 Example

If $$[Z,XY]$$, then the clinic is independent of the drug and the response. In other words, the response of a subject to treatment may depend on the drug they received, but neither of these are associated with the clinic that they went to.

### 3.3.3 Marginal Independence

• $$X$$ and $$Y$$ are marginally independent of $$Z$$ if and only if $$$\pi_{ij+} = \pi_{i++} \pi_{+j+} \qquad i = 1,...,I \quad j = 1,...,J \quad k = 1,...,K \tag{3.3}$$$

• Here, we actually ignore $$Z$$.

• Such marginal independence is symbolised $$[X,Y]$$.

#### 3.3.3.1 Example

• If $$Y$$ and $$Z$$ are marginally independent of $$X$$ (that is $$[Y,Z]$$), then this would imply that response to treatment is not associated with the clinic attended if we ignore which drug was received.

### 3.3.4 Conditional Independence

• Under a multinomial sampling scheme, the joint probabilities of the three-way table cells $$\pi_{ijk}$$ can be expressed in terms of conditional probabilities as $\begin{eqnarray} \pi_{ijk} & = & P(X=i, Y=j, Z=k) \\ & = & P(Y=j| X=i, Z=k) \, P(X=i, Z=k) \\ & = & \pi_{j|ik} \pi_{i+k} \end{eqnarray}$ which under conditional independence of $$X$$ and $$Y$$ given $$Z$$ is equal to $\begin{eqnarray} \pi_{ijk} = \pi_{j|k} \pi_{i+k} & = & P(Y=j|Z=k) P(X=i,Z=k) \nonumber \\ & = & P(X=i,Z=k) \frac{P(Y=j,Z=k)}{P(Z=k)} \nonumber \\ & = & \frac{\pi_{i+k}\pi_{+jk}}{\pi_{++k}} \tag{3.4} \\ && \qquad \qquad i = 1,...,I \quad j = 1,...,J \quad k = 1,...,K \nonumber \end{eqnarray}$

• The above conditional independence of $$X$$ and $$Y$$ given $$Z$$ can be symbolised as $$[XZ,YZ]$$.

• The analysis above assumed that $$Y$$ was the response variable. The conditioning approach with $$X=i$$ as response variable would also lead to Equation (3.4), which is symmetric in terms of $$X$$ and $$Y$$.

• The hypotheses of conditional independence of $$[XY, YZ]$$ and $$[XY,XZ]$$ are formed analogously to Equation (3.4).

#### 3.3.4.1 Example

If $$Y$$ and $$Z$$ are conditionally independent given $$X$$ (that is, $$[XY,XZ]$$), this implies that response to treatment is independent of clinic attended given knowledge of which drug was received.

#### 3.3.4.2 Odds Ratios

• Under conditional independence of $$X$$ and $$Y$$ given $$Z$$ ([XZ,YZ]), the $$XZ$$ local odds ratios conditional on $$Y$$ are equal to the $$XZ$$ marginal local odds ratios, that is41 $$$r_{i(j)k}^{XZ} = r_{ik}^{XZ} \qquad i = 1,...,I-1 \quad j = 1,...,J \quad k = 1,...,K-1 \tag{3.5}$$$ In other words, the marginal and conditional $$XZ$$ associations coincide.

• By symmetry, we also have that $$$r_{(i)jk}^{YZ} = r_{jk}^{YZ} \qquad i = 1,...,I \quad j = 1,...,J-1 \quad k = 1,...,K-1$$$ that is, the marginal and conditional $$YZ$$ associations coincide.

• However, the $$XY$$ marginal and conditional associations do not coincide.

• Such arguments for $$[XY,YZ]$$ and $$[XY,XZ]$$ are analogous.

### 3.3.5 Conditional and Marginal Independence

Conditional independence does not imply marginal independence, and marginal independence does not imply conditional independence.

#### 3.3.5.1 Example

##### 3.3.5.1.1 Marginal but not Conditional Independence
• Suppose response $$Y$$ and clinic $$Z$$ are marginally independent of treatment drug $$X$$. However, there may be a conditional association between response to treatment $$Y$$ and clinic attended $$Z$$ on the drug received $$X$$.

• Example potential explanation: some clinics may be better prepared to care for subjects on some treatment drugs than others, but without knowledge of the treatment drug received, neither clinic is more associated with a successful response.

##### 3.3.5.1.2 Conditional but not Marginal Independence
• Suppose $$Y$$ and $$Z$$ are conditionally independent given $$X$$ (that is, $$[XY,XZ]$$), then this implies that response to treatment is independent of clinic attended given knowledge of which drug was received. However, there may be a marginal association between response to treatment $$Y$$ and clinic attended $$Z$$ if we ignore which treatment drug $$X$$ was received.

• Example potential explanation: Given knowledge of the treatment drug, it does not matter which clinic the subject attends. However, without knowledge of the treatment drug, one clinic may be more associated with a successful response (perhaps their stock of the more successful drug is greater…).

### 3.3.6 Homogeneous Associations

• Homogeneous associations (also known as no three-factor interactions) mean that the conditional relationship between any pair of variables given the third one is the same at each level of the third variable; but not necessarily independent.

• This relation implies that if we know all two-way tables between the three variables, we have sufficient information to compute $$(\pi_{ijk})$$.

• However, there are no separable closed-form estimated for the expected joint probabilities $$(\hat{\pi}_{ijk})$$, hence maximum likelihood estimates must be computed by an iterative procedure such as Iterative Proportional Fitting or Newton-Raphson.

• Such homogeneous associations are symbolised $$[XY, XZ, YZ]$$.

#### 3.3.6.1 Odds Ratios

• Homogeneous associations can be thought of in terms of conditional odds ratios as follows:

• the $$XY$$ partial odds ratios at each level of $$Z$$ are identical: $$r_{ij(k)}^{XY} = r_{ij}^{XY, \star}$$

• the $$XZ$$ partial odds ratios at each level of $$Y$$ are identical: $$r_{i(j)k}^{XZ} = r_{ik}^{XZ, \star}$$

• the $$YZ$$ partial odds ratios at each level of $$X$$ are identical: $$r_{(i)jk}^{YZ} = r_{jk}^{YZ, \star}$$

• Note that $$r_{ij}^{XY, \star}, r_{ik}^{XZ, \star}, r_{jk}^{YZ, \star}$$ are not necessarily the same as the corresponding marginal odds ratios $$r_{ij}^{XY}, r_{ik}^{XZ}, r_{jk}^{YZ}$$.

#### 3.3.6.2 Example

The treatment response and treatment drug have the same association for each clinic.

More precisely, we have $$$r_{1,1,(k)}^{XY} = r_{1,1}^{XY, \star} \iff \frac{\pi_{1,1,(k)}}{ \pi_{1,2,(k)}} = r_{1,1}^{XY, \star} \frac{\pi_{2,1,(k)}}{\pi_{2,2,(k)}} \qquad k = 1,2$$$ which means that each drug has a different odds of success depending on the clinic, however, the odds of treatment success of drug $$A$$ are a fixed constant $$r_{1,1}^{XY, \star}$$ greater than the odds of treatment success of drug $$B$$, regardless of the clinic.

### 3.3.7 Tests for Independence

• Marginal independence (Equation (3.3)) can be tested using the test for independence presented in Section 2.4.3.1 applied on the corresponding two-way marginal table.

• Hypotheses of the independence statements defined by Equations (3.1), (3.2) and (3.4) could be tested analogously using the relevant marginal counts.

• We do not consider these tests, but defer to log-linear models (soon!).

• A specific test of independence of $$XY$$ at each level of $$Z$$ for $$2 \times 2 \times K$$ tables is presented in Section 3.3.10.

### 3.3.8 Summary of Relationships

We present a summary of which independence relationships can be implied from which others, and which can’t, in Figure 3.3.

### 3.3.9 Multi-way Tables

• Analogous definitions of the various types of independence exist for general multi-way tables.

#### 3.3.9.1 Example

We might analyse $$(n_{i_1+(i_3)i_4(i_5)})$$ across the different levels of $$(I_3,I_5)$$ to see if $$(I_1,I_4)$$ and $$(I_3,I_5)$$ are marginally independent of $$I_2$$. We will explore this a bit, but in general, we look to log-linear models.

### 3.3.10 Mantel-Haenszel Test for $$2 \times 2 \times K$$ Tables

• We will discuss the particular case of $$X$$ and $$Y$$ being binary variables that are cross-classified across the $$K$$ layers of a variable $$Z$$, forming $$K$$ $$2 \times 2$$ partial tables $$n_{ij(k)}, \, k = 1,...,K$$.

• The Mantel-Haenszel Test is for testing the conditional independence of $$X$$ and $$Y$$ given $$Z$$ for these $$2 \times 2 \times K$$ tables, that is, it considers the hypotheses $\begin{eqnarray} \mathcal{H}_0: & \, X,Y \textrm{are independent conditional on the level of} Z. \\ \mathcal{H}_1: & \, X,Y \textrm{are not independent conditional on the level of} Z. \end{eqnarray}$ or in other words $\begin{eqnarray} \mathcal{H}_0: & \, r_{12(k)} = 1, \, \textrm{for all} \, k = 1,...,K \\ \mathcal{H}_1: & \, r_{12(k)} \neq 1, \, \textrm{for some} \, k = 1,...,K \\ \end{eqnarray}$

• The Mantel-Haenszel Test conditions on the row and column marginals of each of the $$K$$ partial tables.

• Under $$\mathcal{H}_0$$, every partial table $$n_{11k}$$ follows the hypergeometric distribution42 $$\mathcal{H} g(N = n_{++k}, M = n_{1,+,k}, q = n_{+,1,k})$$43, and thus has mean and variance $$$\hat{E}_{11k} = \frac{n_{1+k} n_{+1k}}{n_{++k}} \qquad \qquad \hat{\sigma}^2_{11k} = \frac{n_{1+k} n_{2+k} n_{+1k} n_{+2k}}{n^2_{++k}(n_{++k} - 1)} \nonumber$$$

• We therefore have that $$\sum_k n_{11k}$$ has mean $$\sum_k \hat{E}_{11k}$$ and variance $$\sum_k \hat{\sigma}^2_{11k}$$, since the partial tables are independent to each other.

• The Mantel–Haenszel test statistic is defined as44 $$$T_{MH} = \frac{[\sum_k (n_{11k} - \hat{E}_{11k})]^2}{\sum_k \hat{\sigma}_{11k}^2} \tag{3.6}$$$

• $$T_{MH}$$ is asymptotically $$\chi^2_1$$ under $$\mathcal{H}_0$$.

• If $$T_{MH(obs)}$$ is the observed value of the test statistic for a particular case, then the $$p$$-value is $$P(\chi_1^2 > T_{MH(obs)})$$.

• When the $$XY$$ association is similar across the partial tables, then the test is more powerful.

• It loses in power when the underlying associations vary across the layers, especially when they are of different direction, since the differences $$n_{11k} - \hat{E}_{11k}$$ will then cancel out in the sum of the statistic given by Equation (3.6).

1. Q3-1 involves showing that Equation (3.5) holds.↩︎

2. See Section 1.9.3.↩︎

3. Why hypergeometric? Well, for any $$2 \times 2$$ table we have $$n_{++k}$$ items. We condition on row and column margins, so we assume knowledge of $$n_{i,+,k}$$ and $$n_{+,j,k}$$. Therefore, this table can be viewed as having a population of $$N = n_{++k}$$. In that population, we know that $$M= n_{1,+,k}$$ of these items are such that $$i=1$$. If the two variables $$X$$ and $$Y$$ are conditionally independent given $$Z$$, then we could view $$N_{1,1,k}$$ to be the result of picking $$q = n_{+,1,k}$$ items (those going into column 1) randomly from $$N = n_{++k}$$, of which $$M=n_{1,+,k}$$ (those in row 1). Therefore $$N_{1,1,k} \sim \mathcal{H} g(N = n_{++k}, M = n_{1,+,k}, q = n_{+,1,k})$$↩︎

4. Note that the square is outside of the summation.↩︎