AMDUNNETT procedure

Forms Dunnett's simultaneous confidence interval around a control (R.W. Payne).


Options

PRINT = string
Controls printed output (interval); default inte

METHOD = string
Form of the alternative hypothesis (twosided, greaterthan, lessthan); default twos

CIPROBABILITY = scalar
Probability level for the confidence interval; default 0.95, i.e. a 95% confidence interval

LOWER = scalar
Saves the lower confidence limit

UPPER = scalar
Saves the upper confidence limit

SAVE = ANOVA save structure
Save structure to provide the means; default uses the save structure from the most recent ANOVA


Parameters

FACTOR = factors
Define the model term whose means are to be compared

CONTROL = scalars or texts
Scalar or single-valued text for each factor to identify which of the means of the term is the control; default uses the reference level of the FACTOR


Description

AMDUNNETT is useful when you want to compare several treatments with a control treatment, and use a critical value that controls the chance that any one comparison may be found significant when there are no true differences. (It is designed thus to take account of the fact that you are making multiple comparisons with the control.)

   The FACTOR parameter lists the factors that define the treatment term whose means are to be compared. The means are usually taken from the most recent analysis of variance (performed by ANOVA), but you can set the SAVE option to a save structure from another ANOVA if you want to examine means from an earlier analysis. The CONTROL parameter specifies a list of scalars to identify the levels of the factors that correspond to the control, or you can use a string (or single-valued text) to identify the level of any factor that has labels. If CONTROL is unset, AMDUNNETT uses the reference level of the FACTOR.

   The METHOD option defines the type of interval that is formed. By default AMDUNNETT forms a two-sided interval. If you set METHOD=lowerthan, a lower confidence interval is formed to assess the one-sided test of the null hypothesis that the treatment means are not lower than the control mean. Alternatively, you can set METHOD=greaterthan, to obtain an upper confidence interval to assess the one-sided test of the null hypothesis that the treatment means are not greater than the mean of the control.

   The probability for the confidence interval is specified by the CIPROBABILITY option; the default 0.95 gives a 95% interval. The lower and upper values of the interval can be saved (in scalars) using the LOWER and UPPER options, respectively. By default the interval is printed, but this can be suppressed by setting option PRINT=*.

 

Options: PRINT, METHOD, CIPROBABILITY, LOWER, UPPER, SAVE.

Parameters: FACTOR, CONTROL.


Method

AMDUNNETT uses the methods of Dunnett (1955, 1989); also see Hsu (1996) Chapter 3.


Action with RESTRICT

If the Y variate in the original ANOVA was restricted, only the units not excluded by the restriction will have been analysed.


References

Dunnett, C.W. (1955). A multiple comparison procedure for comparing several treatments with a control. Journal of the Americal Statistical Association, 50, 1096-1121.

Dunnett, C.W. (1989). Algorithm AS251 Multivariate normal probability intervals with product correlation structure. Applied Statistics, 38, 564-579.

Hsu, J.C. (1996). Multiple Comparisons Theory and Methods. Chapman & Hall, London.