graph twoway qfit — Twoway quadratic prediction plots

Title stata.com

graph twoway qﬁt — Twoway quadratic prediction plots

Syntax Menu Description Options

Remarks and examples Also see

Syntax

twoway qfit yvar xvar



 

weight

 

, options

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options Description

range(# #) range over which predictions calculated

n(#) number of prediction points

atobs calculate predictions at xvar

estopts(regress options) options for regress

predopts(predict options) options for predict

cline options change look of predicted line

axis choice options associate plot with alternative axis

twoway options titles, legends, axes, added lines and text, by, regions, name,

aspect ratio, etc.

See [G-3] cline options, [G-3] axis choice options, and [G-3] twoway options.

All options are rightmost; see [G-4] concept: repeated options.

yvar and xvar may contain time-series operators; see [U] 11.4.4 Time-series varlists.

aweights, fweights, and pweights are allowed. Weights, if speciﬁed, affect estimation but not how

the weighted results are plotted. See [U] 11.1.6 weight.

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

Description

twoway qfit calculates the prediction for yvar from a linear regression of yvar on xvar and xvar

and plots the resulting curve.

Options

range(# #) speciﬁes the x range over which predictions are calculated. The default is range(. .),

meaning the minimum and maximum values of xvar. range(0 10) would make the range 0 to

10, range(. 10) would make the range the minimum to 10, and range(0 .) would make the

range 0 to the maximum.

2 graph twoway qﬁt — Twoway quadratic prediction plots

n(#) speciﬁes the number of points at which predictions over range() are to be calculated. The

default is n(100).

atobs is an alternative to n(). It speciﬁes that the predictions be calculated at the xvar values. atobs

is the default if predopts() is speciﬁed and any statistic other than xb is requested.

estopts(regress options) speciﬁes options to be passed along to regress to estimate the linear

regression from which the curve will be predicted; see [R] regress. If this option is speciﬁed,

commonly speciﬁed is estopts(nocons).

predopts(predict

options) speciﬁes options to be passed along to predict to obtain the predictions

after estimation by regress; see [R] regress postestimation.

cline options specify how the prediction line is rendered; 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 and examples stata.com

Remarks are presented under the following headings:

Typical use

Cautions

Use with by( )

Typical use

twoway qfit is nearly always used in conjunction with other twoway plottypes, such as

. use http://www.stata-press.com/data/r13/auto

(1978 Automobile Data)

. scatter mpg weight || qfit mpg weight

10 20 30 40

2,000 3,000 4,000 5,000

Weight (lbs.)

Mileage (mpg) Fitted values

graph twoway qﬁt — Twoway quadratic prediction plots 3

Results are visually the same as typing

. generate tempvar = weight^2

. regress mpg weight tempvar

. predict fitted

. scatter mpg weight || line fitted weight

Cautions

Do not use twoway qfit when specifying the axis scale options

yscale(log) or xscale(log) to create log scales. Typing

. scatter mpg weight, xscale(log) || qfit mpg weight

produces something that is not a parabola because the regression estimated for the prediction was for

mpg on weight and weight^2, not mpg on log(weight) and log(weight)^2.

Use with by( )

qfit may be used with by() (as can all the twoway plot commands):

. scatter mpg weight || qfit mpg weight ||, by(foreign, total row(1))

10 20 30 40

2,000 3,000 4,000 5,000 2,000 3,000 4,000 5,000 2,000 3,000 4,000 5,000

Domestic Foreign Total

Mileage (mpg) Fitted values

Weight (lbs.)

Graphs by Car type

Also see

[G-2] graph twoway line — Twoway line plots

[G-2] graph twoway lﬁt — Twoway linear prediction plots

[G-2] graph twoway fpﬁt — Twoway fractional-polynomial prediction plots

[G-2] graph twoway mband — Twoway median-band plots

[G-2] graph twoway mspline — Twoway median-spline plots

[G-2] graph twoway qﬁtci — Twoway quadratic prediction plots with CIs

[R] regress — Linear regression