NAG Library Routine Document

G11AAF

are the fitted values from the model that assumes the effects due to the classification variables are additive, i.e., there is no association. These values are the expected cell frequencies and are given by

f_{i j} = n_{i .} n_{. j} / n .

Under the hypothesis of no association between the two classification variables, both these statistics have, approximately, a

χ^{2}

-distribution with

(c - 1) (r - 1)

degrees of freedom. This distribution is arrived at under the assumption that the expected cell frequencies,

f_{i j}

, are not too small. For a discussion of this point see Everitt (1977). He concludes by saying, ‘... in the majority of cases the chi-square criterion may be used for tables with expectations in excess of

0.5

in the smallest cell’.

In the case of the

2 \times 2

table, i.e.,

c = 2

and

r = 2

, the

χ^{2}

approximation can be improved by using Yates' continuity correction factor. This decreases the absolute value of

(n_{i j} - f_{i j})

\frac{1}{2}

. For

2 \times 2

tables with a small value of

n

the exact probabilities from Fisher's test are computed. These are based on the hypergeometric distribution and are computed using G01BLF. A two tail probability is computed as

\min (1, 2 p_{u}, 2 p_{l})

, where

p_{u}

and

p_{l}

are the upper and lower one-tail probabilities from the hypergeometric distribution.

4 References

Everitt B S (1977) The Analysis of Contingency Tables Chapman and Hall

Kendall M G and Stuart A (1973) The Advanced Theory of Statistics (Volume 2) (3rd Edition) Griffin

5 Parameters

1: $NROW$ – INTEGERInput: On entry: $r$ , the number of rows in the contingency table.

Constraint: $NROW \geq 2$ .
2: $NCOL$ – INTEGERInput: On entry: $c$ , the number of columns in the contingency table.

Constraint: $NCOL \geq 2$ .
3: $NOBS (LDNOBS, NCOL)$ – INTEGER arrayInput: On entry: the contingency table $NOBS (i, j)$ must contain $n_{i j}$ , for $i = 1, 2, \dots, r$ and $j = 1, 2, \dots, c$ .

Constraint: $NOBS (i, j) \geq 0$ , for $i = 1, 2, \dots, r$ and $j = 1, 2, \dots, c$ .
4: $LDNOBS$ – INTEGERInput: On entry: the first dimension of the arrays NOBS, EXPT and CHIST as declared in the (sub)program from which G11AAF is called.

Constraint: $LDNOBS \geq NROW$ .
5: $EXPT (LDNOBS, NCOL)$ – REAL (KIND=nag_wp) arrayOutput: On exit: the table of expected values. $EXPT (i, j)$ contains $f_{i j}$ , for $i = 1, 2, \dots, r$ and $j = 1, 2, \dots, c$ .
6: $CHIST (LDNOBS, NCOL)$ – REAL (KIND=nag_wp) arrayOutput: On exit: the table of $χ^{2}$ contributions. $CHIST (i, j)$ contains $\frac{{(n_{i j} - f_{i j})}^{2}}{f_{i j}}$ , for $i = 1, 2, \dots, r$ and $j = 1, 2, \dots, c$ .
7: $PROB$ – REAL (KIND=nag_wp)Output: On exit: if $c = 2$ , $r = 2$ and $n \leq 40$ then PROB contains the two tail significance level for Fisher's exact test, otherwise PROB contains the significance level from the Pearson $χ^{2}$ statistic.
8: $CHI$ – REAL (KIND=nag_wp)Output: On exit: the Pearson $χ^{2}$ statistic.
9: $G$ – REAL (KIND=nag_wp)Output: On exit: the likelihood ratio test statistic.
10: $DF$ – REAL (KIND=nag_wp)Output: On exit: the degrees of freedom for the statistics.
11: $IFAIL$ – INTEGERInput/Output: On entry: IFAIL must be set to $0$ , $- 1 or 1$ . If you are unfamiliar with this parameter you should refer to Section 3.3 in the Essential Introduction for details.
For environments where it might be inappropriate to halt program execution when an error is detected, the value $- 1 or 1$ is recommended. If the output of error messages is undesirable, then the value $1$ is recommended. Otherwise, because for this routine the values of the output parameters may be useful even if $IFAIL \neq 0$ on exit, the recommended value is $- 1$ . When the value $- 1 or 1$ is used it is essential to test the value of IFAIL on exit.

On exit: $IFAIL = 0$ unless the routine detects an error or a warning has been flagged (see Section 6).

6 Error Indicators and Warnings

If on entry

IFAIL = 0

- 1

, explanatory error messages are output on the current error message unit (as defined by X04AAF).

Note: G11AAF may return useful information for one or more of the following detected errors or warnings.

Errors or warnings detected by the routine:

$IFAIL = 1$

On entry,	$NROW < 2$ ,
or	$NCOL < 2$ ,
or	$LDNOBS < NROW$ .

$IFAIL = 2$

On entry,

a value in

NOBS < 0

, or all values in NOBS are zero.

$IFAIL = 3$

On entry,

2 \times 2

table has a row or column with both values

0

$IFAIL = 4$: At least one cell has expected frequency, $f_{i j}$ , $\leq 0.5$ . The $χ^{2}$ approximation may be poor.

$IFAIL = - 99$: An unexpected error has been triggered by this routine. Please contact NAG.
See Section 3.8 in the Essential Introduction for further information.

$IFAIL = - 399$: Your licence key may have expired or may not have been installed correctly.
See Section 3.7 in the Essential Introduction for further information.

$IFAIL = - 999$: Dynamic memory allocation failed.
See Section 3.6 in the Essential Introduction for further information.

7 Accuracy

For the accuracy of the probabilities for Fisher's exact test see G01BLF.

8 Parallelism and Performance

Not applicable.

9 Further Comments

The routine G01AFF allows for the automatic amalgamation of rows and columns. In most circumstances this is not recommended; see Everitt (1977).

Multidimensional contingency tables can be analysed using log-linear models fitted by G02GBF.

10 Example

The data below, taken from Everitt (1977), is from

141

patients with brain tumours. The row classification variable is the site of the tumour: frontal lobes, temporal lobes and other cerebral areas. The column classification variable is the type of tumour: benign, malignant and other cerebral tumours.

\begin{array}{r} 23 & 9 & 6 & 38 \\ 21 & 4 & 3 & 28 \\ 34 & 24 & 17 & 75 \\ 78 & 37 & 26 & 141 \end{array}

The data is read in and the statistics computed and printed.

NAG Library Routine DocumentG11AAF

▸▿ Contents

1 Purpose

2 Specification

3 Description