Stata 15 help for twoway kdensity

[G-2] graph twoway kdensity -- Kernel density plots

Syntax

twoway kdensity varname [if] [in] [weight] [, options]

options Description ------------------------------------------------------------------------- bwidth(#) smoothing parameter kernel(kernel) specify kernel function; default is kernel(epanechnikov)

range(# #) range for plot, minimum and maximum range(varname) range for plot obtained from varname n(#) number of points to evaluate area(#) rescaling parameter horizontal graph horizontally boundary estimate density one bwidth() beyond maximum and minimum; not allowed with range()

cline_options change look of the line

axis_choice_options associate plot with alternative axis

twoway_options titles, legends, axes, added lines and text, by, regions, name, aspect ratio, etc. -------------------------------------------------------------------------

kernel Description ------------------------------------------------------------------------- epanechnikov Epanechnikov kernel function; the default epan2 alternative Epanechnikov kernel function biweight biweight kernel function cosine cosine trace kernel function gaussian Gaussian kernel function parzen Parzen kernel function rectangle rectangle kernel function triangle triangle kernel function -------------------------------------------------------------------------

fweights and aweights are allowed; see weight.

Menu

Graphics > Twoway graph (scatter, line, etc.)

Description

graph twoway kdensity plots a kernel density estimate for varname using graph twoway line.

Options

bwidth(#) and kernel(kernel) specify how the kernel density estimate is to be obtained and are in fact the same options as those specified with the command kdensity; see [R] kdensity.

bwidth(#) specifies the smoothing parameter.

kernel(kernel) specify the kernel-weight function to be used. The default is kernel(epanechnikov).

See [R] kdensity for more information about these options.

All the other graph twoway kdensity options modify how the result is displayed, not how it is obtained.

range(# #) and range(varname) specify the range of values at which the kernel density estimates are to be plotted. The default is range(m M), where m and M are the minimum and maximum of the varname specified on the graph twoway kdensity command.

range(# #) specifies a pair of numbers to be used as the minimum and maximum.

range(varname) specifies another variable for which its minimum and maximum are to be used.

n(#) specifies the number of points at which the estimate is evaluated. The default is n(300).

area(#) specifies a multiplier by which the density estimates are adjusted before being plotted. The default is area(1). area() is useful when overlaying a density estimate on top of a histogram that is itself not scaled as a density. For instance, if you wished to scale the density estimate as a frequency, area() would be specified as the total number of nonmissing observations.

horizontal specifies that the result be plotted horizontally (that is, reflected along the identity line).

boundary specifies that the result be estimated for one bwidth() beyond the maximum and minimum value of varname. boundary cannot be specified with range().

cline_options specify how the density line is rendered and its appearance; see [G-3] cline_options.

axis_choice_options associate the plot with a particular y or x axis on the graph; see [G-3] axis_choice_options.

twoway_options are a set of common options supported by all twoway graphs. These options allow you to title graphs, name graphs, control axes and legends, add lines and text, set aspect ratios, create graphs over by() groups, and change some advanced settings. See [G-3] twoway_options.

Remarks

graph twoway kdensity varname uses the kdensity command to obtain an estimate of the density of varname and uses graph twoway line to plot the result.

Remarks are presented under the following headings:

Typical use Use with by()

Typical use

The density estimate is often graphed on top of the histogram:

. sysuse lifeexp

. twoway histogram lexp, color(*.5) || kdensity lexp (click to run)

Notice the use of graph twoway histogram's color(*.5) option to dim the bars and make the line stand out; see [G-4] colorstyle.

Notice also the y and x axis titles: "Density/kdensity lexp" and "Life expectancy at birth/x". The "kdensity lexp" and "x" were contributed by the twoway kdensity. When you overlay graphs, you nearly always need to respecify the axis titles using the axis_title_options ytitle() and xtitle(); see [G-3] axis_title_options.

Use with by()

graph twoway kdensity may be used with by():

. sysuse lifeexp, clear

. twoway histogram lexp, color(*.5) || kdensity lexp ||, by(region) (click to run)


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