# 2D Box Plots - Quick Tab

Graphical Analytic Techniques

Use the options on the Quick tab of the 2D Box Plots Startup Panel to quickly specify the variables and options to use in creating a box plot. For more options, use the Advanced tab.

Graph type. By default, only a Box-Whiskers or High-Low Close plot can be selected from this box. More graph types are available on the Advanced tab. Regular Multiple

Variables. Click this button to display a standard variable selection dialog box, in which you can select the grouping (category) and dependent variables for the graph. If you select a category variable and more than one dependent variable for Regular box plots (see the Graph type, above), a sequence of graphs (one for each dependent variable) will be produced; if the Graph type is set to Multiple, box plots for all selected variables will be combined in one graph. The selection that you make will then be displayed in the area of the dialog below the Variables button.

Note that if you select only one dependent variable and no category variable, a single box plot representing the distribution of the dependent variable will be produced. If you select multiple dependent variables and no category variable, a single graph with box plots for each selected variable will be produced. This latter plot is useful for comparing the distribution of several variables.

Grouping Intervals. Use the options in this group box to select a method of categorization for the selected variable(s). Each of the methods is discussed in Method of Categorization.

Graph icon. The graph icon in the middle of the tab represents the currently selected Graph Type, and the selection of values for the Box, Middle Point, and Whiskers on the Advanced tab, if changed from the default. The graph icon previews these three selections and the specific statistics that define the current box plot.

Middle point. The options in this group box control the type of value and appearance of the middle point.

Value. The middle point can be either the Mean, Median, Mean/Median (uses the Mean as the middle point, plus it has an added marker for the Median), or Median/Mean (uses the Median as the middle point, plus it has an added marker for the Mean) of the selected variable. The options available for the Box and Whiskers on the Advanced tab depend on this selection.

Style. Use the Style box to control how the middle point is represented (by a Line or Point).

Pooled variance. This option is available when you select Mean as the Midpoint (see above). The setting of this check box determines how the standard deviations and standard errors (for the means) are computed from grouped data. When this check box is selected, STATISTICA will compute the pooled within-group (category) variance for all groups (categories), and use this value as an estimate of s (sigma) when computing the standard errors for the means (see, for example, Milliken and Johnson, 1984). Specifically, the program will compute the pooled within-group (category) variance as:

spooled2 = 1/(n-k) * [s12 *(n1 -1) + ... + sk2 *(nk -1)]

In this equation, k refers to the k groups in the plot, si2, refers to the variance in the i'th category or group, ni refers to number of valid observations in the i'th category or group, and n is the overall number of valid observations in the plot.

The standard error of the mean for the i'th group is then computed as:

s.e.(mean) = spooled /square root(ni)