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The ANOVA Procedure |

**MEANS***effects <***/**options >**;**

PROC ANOVA can compute means of the dependent variables for any effect that appears on the right-hand side in the MODEL statement.

You can use any number of MEANS statements, provided that they appear after the MODEL statement. For example, suppose A and B each have two levels. Then, if you use the following statements

proc anova; class A B; model Y=A B A*B; means A B / tukey; means A*B; run;

means, standard deviations, and Tukey's multiple comparison tests are produced for each level of the main effects A and B, and just the means and standard deviations for each of the four combinations of levels for A*B. Since multiple comparisons options apply only to main effects, the single MEANS statement

means A B A*B / tukey;

produces the same results.

Options are provided to perform multiple comparison tests for only main effects in the model. PROC ANOVA does not perform multiple comparison tests for interaction terms in the model; for multiple comparisons of interaction terms, see the LSMEANS statement in

Chapter 30, "The GLM Procedure." The following table summarizes categories of options available in the MEANS statement.

Task |
Available options |

Perform multiple comparison tests | BON |

DUNCAN | |

DUNNETT | |

DUNNETTL | |

DUNNETTU | |

GABRIEL | |

GT2 | |

LSD | |

REGWQ | |

SCHEFFE | |

SIDAK | |

SMM | |

Perform multiple comparison tests | SNK |

T | |

TUKEY | |

WALLER | |

Specify additional details for | ALPHA= |

multiple comparison tests | CLDIFF |

CLM | |

E= | |

KRATIO= | |

LINES | |

NOSORT | |

Test for homogeneity of variances | HOVTEST |

Compensate for heterogeneous variances | WELCH |

Descriptions of these options follow. For a further discussion of these options, see the section "Multiple Comparisons" in Chapter 30, "The GLM Procedure."

**ALPHA=***p*-
specifies the level of significance for comparisons among the means.
By default, ALPHA=0.05.
You can specify any value greater than 0 and less than 1.
**BON**-
performs Bonferroni
*t*tests of differences between means for all main effect means in the MEANS statement. See the CLDIFF and LINES options, which follow, for a discussion of how the procedure displays results. **CLDIFF**-
presents results of the BON, GABRIEL, SCHEFFE, SIDAK,
SMM, GT2, T, LSD, and TUKEY options as confidence
intervals for all pairwise differences between means,
and the results of the DUNNETT, DUNNETTU, and DUNNETTL
options as confidence intervals for differences with the control.
The CLDIFF option is the default for unequal cell sizes unless
the DUNCAN, REGWQ, SNK, or WALLER option is specified.
**CLM**-
presents results of the BON, GABRIEL, SCHEFFE, SIDAK, SMM,
T, and LSD options as intervals for the mean of
each level of the variables specified in the MEANS statement.
For all options except GABRIEL, the intervals are confidence
intervals for the true means. For the GABRIEL option,
they are
*comparison intervals*for comparing means pairwise: in this case, if the intervals corresponding to two means overlap, the difference between them is insignificant according to Gabriel's method. **DUNCAN**-
performs Duncan's multiple range test on all
main effect means given in the MEANS statement.
See the LINES option for a discussion
of how the procedure displays results.
**DUNNETT < (***formatted-control-values*) >-
performs Dunnett's two-tailed
*t*test, testing if any treatments are significantly different from a single control for all main effects means in the MEANS statement.

To specify which level of the effect is the control, enclose the formatted value in quotes in parentheses after the keyword. If more than one effect is specified in the MEANS statement, you can use a list of control values within the parentheses. By default, the first level of the effect is used as the control. For example,means a / dunnett('CONTROL');

where CONTROL is the formatted control value of A. As another example,means a b c / dunnett('CNTLA' 'CNTLB' 'CNTLC');

where CNTLA, CNTLB, and CNTLC are the formatted control values for A, B, and C, respectively. **DUNNETTL < (***formatted-control-value*) >-
performs Dunnett's one-tailed
*t*test, testing if any treatment is significantly less than the control. Control level information is specified as described previously for the DUNNETT option. **DUNNETTU < (***formatted-control-value*) >-
performs Dunnett's one-tailed
*t*test, testing if any treatment is significantly greater than the control. Control level information is specified as described previously for the DUNNETT option. **E=***effect*-
specifies the error mean square used in the multiple comparisons.
By default, PROC ANOVA uses the residual Mean Square (MS).
The effect specified with the E= option must be a term in
the model; otherwise, the procedure uses the residual MS.
**GABRIEL**-
performs Gabriel's multiple-comparison procedure
on all main effect means in the MEANS statement.
See the CLDIFF and LINES options for discussions
of how the procedure displays results.
**GT2**- see the SMM option.
**HOVTEST****HOVTEST=BARTLETT****HOVTEST=BF****HOVTEST=LEVENE <(TYPE=ABS | SQUARE)>****HOVTEST=OBRIEN <(W=***number*)>-
requests a homogeneity of variance test for the groups defined by the
MEANS effect. You can optionally specify a particular test; if you do
not specify a test, Levene's test (Levene 1960) with TYPE=SQUARE is
computed. Note that this option is ignored unless your MODEL statement
specifies a simple one-way model.

The HOVTEST=BARTLETT option specifies Bartlett's test (Bartlett 1937), a modification of the normal-theory likelihood ratio test.

The HOVTEST=BF option specifies Brown and Forsythe's variation of Levene's test (Brown and Forsythe 1974).

The HOVTEST=LEVENE option specifies Levene's test (Levene 1960), which is widely considered to be the standard homogeneity of variance test. You can use the TYPE= option in parentheses to specify whether to use the absolute residuals (TYPE=ABS) or the squared residuals (TYPE=SQUARE) in Levene's test. The default is TYPE=SQUARE.

The HOVTEST=OBRIEN option specifies O'Brien's test (O'Brien 1979), which is basically a modification of HOVTEST=LEVENE(TYPE=SQUARE). You can use the W= option in parentheses to tune the variable to match the suspected kurtosis of the underlying distribution. By default, W=0.5, as suggested by O'Brien (1979, 1981).

See the section "Homogeneity of Variance in One-Way Models" in Chapter 30, "The GLM Procedure," for more details on these methods. Example 30.10 in the same chapter illustrates the use of the HOVTEST and WELCH options in the MEANS statement in testing for equal group variances. **KRATIO=***value*-
specifies the Type 1/Type 2 error seriousness
ratio for the Waller-Duncan test.
Reasonable values for KRATIO are 50, 100, and 500,
which roughly correspond for the two-level
case to ALPHA levels of 0.1, 0.05, and 0.01.
By default, the procedure uses the default value of 100.
**LINES**-
presents results of the BON, DUNCAN, GABRIEL,
REGWQ, SCHEFFE, SIDAK, SMM, GT2, SNK, T, LSD, TUKEY,
and WALLER options by listing the means in descending
order and indicating nonsignificant subsets by line
segments beside the corresponding means.
The LINES option is appropriate for equal cell sizes,
for which it is the default.
The LINES option is also the default if the DUNCAN,
REGWQ, SNK, or WALLER option is specified, or if
there are only two cells of unequal size.
If the cell sizes are unequal, the harmonic mean of
the cell sizes is used, which may lead to somewhat
liberal tests if the cell sizes are highly disparate.
The LINES option cannot be used in combination with the
DUNNETT, DUNNETTL, or DUNNETTU option.
In addition, the procedure has a restriction that no
more than 24 overlapping groups of means can exist.
If a mean belongs to more than 24 groups,
the procedure issues an error message.
You can either reduce the number of levels of the variable
or use a multiple comparison test that allows the
CLDIFF option rather than the LINES option.
**LSD**-
see the T option.
**NOSORT**-
prevents the means from being sorted into descending order
when the CLDIFF or CLM option is specified.
**REGWQ**-
performs the Ryan-Einot-Gabriel-Welsch multiple range
test on all main effect means in the MEANS statement.
See the LINES option for a discussion
of how the procedure displays results.
**SCHEFFE**-
performs Scheff's multiple-comparison procedure
on all main effect means in the MEANS statement.
See the CLDIFF and LINES options for discussions
of how the procedure displays results.
**SIDAK**-
performs pairwise
*t*tests on differences between means with levels adjusted according to Sidak's inequality for all main effect means in the MEANS statement. See the CLDIFF and LINES options for discussions of how the procedure displays results. **SMM****GT2**-
performs pairwise comparisons based on the studentized
maximum modulus and Sidak's uncorrelated-
*t*inequality, yielding Hochberg's GT2 method when sample sizes are unequal, for all main effect means in the MEANS statement. See the CLDIFF and LINES options for discussions of how the procedure displays results. **SNK**-
performs the Student-Newman-Keuls multiple range test on all
main effect means in the MEANS statement. See the
LINES option for a
discussion of how the procedure displays results.
**T****LSD**-
performs pairwise
*t*tests, equivalent to Fisher's least-significant-difference test in the case of equal cell sizes, for all main effect means in the MEANS statement. See the CLDIFF and LINES options for discussions of how the procedure displays results. **TUKEY**-
performs Tukey's studentized range test (HSD)
on all main effect means in the MEANS statement.
(When the group sizes are different, this is the Tukey-Kramer test.)
See the CLDIFF and LINES options for discussions
of how the procedure displays results.
**WALLER**-
performs the Waller-Duncan
*k*-ratio*t*test on all main effect means in the MEANS statement. See the KRATIO= option for information on controlling details of the test, and see the LINES option for a discussion of how the procedure displays results. **WELCH**-
requests Welch's (1951) variance-weighted one-way ANOVA. This
alternative to the usual analysis of variance for a one-way model is
robust to the assumption of equal within-group variances. This option
is ignored unless your MODEL statement specifies a simple
one-way model.

Note that using the WELCH option merely produces one additional table consisting of Welch's ANOVA. It does not affect all of the other tests displayed by the ANOVA procedure, which still require the assumption of equal variance for exact validity.

See the "Homogeneity of Variance in One-Way Models" section in Chapter 30, "The GLM Procedure," for more details on Welch's ANOVA. Example 30.10 in the same chapter illustrates the use of the HOVTEST and WELCH options in the MEANS statement in testing for equal group variances.

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