### Stata FAQ How can I plot ANOVA cell means in Stata?

There are two ways in which you can plot ANOVA cell means:

1. anovaplot -- user written program (findit anovaplot) or
2. enter a sequence of Stata commands from the command line.

Now, let's read in an example dataset, crf24, adapted from Kirk (1968, First Edition).

use http://www.ats.ucla.edu/stat/stata/faq/crf24

anova y a b a*b

Number of obs =      32     R-squared     =  0.9214
Root MSE      = .877971     Adj R-squared =  0.8985

Source |  Partial SS    df       MS           F     Prob > F
-----------+----------------------------------------------------
Model |         217     7          31      40.22     0.0000
|
a |       3.125     1       3.125       4.05     0.0554
b |       194.5     3  64.8333333      84.11     0.0000
a*b |      19.375     3  6.45833333       8.38     0.0006
|
Residual |        18.5    24  .770833333
-----------+----------------------------------------------------
Total |       235.5    31  7.59677419   

Now, let's plot the cell means.

anovaplot b a, scatter(msym(none))


The graph illustrates the interaction effects in the 2 x 4 factorial ANOVA. Now, let's look at the sequence of Stata commands which can be used to produce these graphs.

predict yhat
sort a b
graph twoway scatter yhat b, connect(L)

In order to do this plot of the cell means it is necessary to predict the cell means using predict yhat. Next the data are sorted on a and b and then yhat is graphed versus b with the points connected by a line for each level of a (the c(L) option: Note the upper case "L").

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