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Smoother graph with confidence levels

phillby

Community Trekker

Joined:

Sep 28, 2011

I have attached a couple of images of graphs I have created.

The raw data is a series of temperature probe measurements showing cooling curves by pallet. The image 'data trends' shows each trend, the image 'smoother' shows a summary curve. I want to be able to represent the distribution on this summary curve. A suggestion has been to have a shaded area around the line that indicates 95% confidence, but we have no idea how to construct this graph. Any ideas?

Phill

3 REPLIES
mastropi

Staff

Joined:

Sep 6, 2011

Hello Phill,

If you are asking how to generate a shaded area, you could use the BAND statement in PROC SGPLOT which is part of ODS graphics.

Here is an example:

ods graphics on;

data toplot;

    set sashelp.air;

    L95 = air - 0.30*air;

    U95 = air + 0.30*air;

run;

proc sgplot data=toplot;

    band    x=date lower=L95 upper=U95 / fillattrs=GraphConfidence legendlabel="Confidence" name="band";

    series  x=date y=air / legendlabel="Summary Curve" name="series";

    keylegend "series" "band"/ location=outside position=bottom;

run;

Note that the order of the BAND and the SERIES statements is important. If reverted, the line produced by the series is not seen.

HTH

Daniel

XanGregg

Staff

Joined:

Jun 23, 2011

I'm not aware of a good definition of a confidence interval for a spline smoother. (There is no real parameter being estimated.) Your data may suggest a definition -- it looks like it's made up of multiple series. For instance, you might take the 95% confidence interval of the mean for each X. With some effort, you can plot such an interval in GB using the Area element in Range mode. You have to make a plot with 3 Ys (data, upper and lower) and use the right-click > element > Y submenu to turn off the data Y for the Area element and turn off the upper/lower Ys for the Smoother element.

2535_smootherinterval.png

Another technique is to make lots of copies of the variable using random samples, and plot smoothers for the copies transparently. Here's an attempt at that with 30 resamples:

2537_smootherinterval2.png

phillby

Community Trekker

Joined:

Sep 28, 2011

Thanks for the ideas. We'll have a look at whether we can apply this to what we're doing. Phill