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From |
"Nick Cox" <n.j.cox@durham.ac.uk> |

To |
<statalist@hsphsun2.harvard.edu> |

Subject |
st: -matrixof- available from SSC |

Date |
Sat, 26 Oct 2002 19:59:06 +0100 |

Thanks to Kit Baum, a new package -matrixof- is now available on SSC for Stata 7 users. In an up-to-date Stata, you can get a capsule description by typing . ssc desc matrixof although if you are interested, what follows says more. You can install by typing . ssc inst matrixof The -matrixof- package contains two commands, -matrixof- and -vectorof-. -matrixof- and -vectorof- are another take on a perennial problem, how to get a lot of similar results at once with as little typing (and as little thinking) as possible. Naturally, many commands and many constructs in Stata have been offered to help here, both as part of official Stata and as user-written programs. -foreach- and -forvalues- are perhaps the most general constructs, and although it may not be obvious from this account, -matrixof- and -vectorof- are merely wrappers for various calls to -foreach- and -forvalues-. As the names should imply, they apply when the result needed in one matrix or vector of numbers -- except that we stretch to doing this also for a matrix or vector of graphs. Enough preamble. Let's cut to some examples. You are thinking correlations, but rank correlations, and you check out the syntax for -ktau- or -spearman-. Frustration, as these niggardly commands will give you results for only one pair of variables at time! You could grit your teeth and type commands one at a time, but you know that Stata should be doing this for you. You could brush up your -foreach- and cycle through a set of variables, once as rows and once as columns. You could think in terms of putting results in a matrix, because then -matrix list- can be used to show the results concisely and tidily. That suggests initialising a matrix and then replacing each element after a call to -ktau-. There are other wrinkles to take care of, such as being consistent when faced with missing values in some variables but not others, and so on. The code to do all this from first principles is not long, but -matrixof- can do it in one line: . matrixof ktau price-foreign, r(tau_b) format(%4.3f) We want a _matrix of_ results from -ktau-. The matrix will pick up all the values from r(tau_b) and we are going to display it with 3 decimal places. Similarly, if we want variations on what -correlate- provides, such as a different format . matrixof correlate price-foreign, r(rho) format(%3.2f) or listwise results . matrixof correlate price-foreign, r(rho) format(%3.2f) listwise . matrixof correlate price-foreign, r(N) listwise then -matrixof- can help. (Note in passing that other Stata programs exist for some of these needs.) Or suppose you want just one column of a correlation matrix, natural given some possible covariates and one response variable. This could be a job for -vectorof- as the results compose a vector . vectorof correlate price-foreign, r(rho) y(mpg) format(%4.3f) Note here the use of a -y()- option: we imagine substituting -mpg- as a variable paired with each of -price-foreign-. Similarly, we might want just a block of a larger correlation matrix. Imagine calculating principal components . factor x1-x6, pc . score pc1-pc6 after which we should want to look at the correlations between the original variables and the PCs. . correlate x1-x6 pc1-pc6 is likely to seem unwieldy -- and the matrix for a larger number of variables and components will surely seem worse. Also, there is no scientific interest in seeing that different components have correlation zero, although at one level it is reassuring. The result needed is . matrixof correlate x1-x6, cols(pc1-pc6) r(rho) format(%4.3f) The idea is that the variable list supplied with this syntax defines the rows of the matrix and that supplied by -cols()- defines the columns. When not asked to pick up an rclass result, or an eclass result, or a global result, left in its wake by some command, -matrixof- and -vectorof- expect to be doing graphics. Say you want to _compare_ graphs, for which it helps to be able to see the graphs within one window. The classic way to do this in Stata is to save the graphs in files g1, g2, g3, ... and then type . graph using g1 g2 g3 ... and unless you have a monitor half the size of Texas you might find it helpful to change the text size first: . set textsize 140 -vectorof- will let you see a series of univariate graphs all at once . vectorof dotplot price-displacement . vectorof quantile price-displacement and you can also use -vectorof- to get bivariate graphs with the same x variable . vectorof graph arithmetic algebra geometry calculus, x(age) or indeed the same y variable, using the -y()- option. -matrixof- allows a display of pairwise graphs. Bug reports to me please. Nick n.j.cox@durham.ac.uk * * For searches and help try: * http://www.stata.com/support/faqs/res/findit.html * http://www.stata.com/support/statalist/faq * http://www.ats.ucla.edu/stat/stata/

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